Process of producing foamed molding from expanded polypropylene resin beads and process of producing expanded polypropylene resin beads

ABSTRACT

A process of producing a foamed molding, wherein expanded, substantially non-crosslinked polypropylene-based resin beads are heated in a mold to fuse-bond the beads together into a unitary body. Each of the expanded beads has been surface-modified with an organic peroxide.

TECHNICAL FIELD

[0001] This invention relates to a process of producing a molded articlefrom expanded polypropylene resin beads and to a process of producingexpanded polypropylene resin beads.

BACKGROUND ART

[0002] A polypropylene resin is now increasingly utilized in variousfields because of excellent mechanical strengths, heat resistance,machinability, cost balance, combustibility and recyclability thereof.Foamed, non-crosslinked resin moldings of a base resin including apolypropylene resin (hereinafter referred to simply as “PP moldings”),which retain the above excellent properties and which have excellentadditional characteristics such as cushioning property and heatresistance, are thus utilized for various applications as packagingmaterials, construction materials, heat insulation materials, etc.

[0003] Recently, there is an increasing demand for PP moldings havinghigher rigidity and lighter weight than the conventional ones. Forexample, in the field of vehicles such as automobiles, PP moldings havebeen used in various parts such as bumper cores, door pats, pillars,tool boxes and floor mats. In view of protection of environment andsaving of energy, light weight and high rigidity PP moldings retainingexcellent cushioning and shock absorbing properties are desired. In thefield of containers and boxes for storing and transporting foods such asfish, molded polystyrene foams have been hitherto used. Because ofinferior shock and heat resistance, however, molded polystyrene foamsare not suitably reused. Therefore, the need for light weight and highrigidity PP moldings is also increasing in this field.

[0004] One known method for improving rigidity of PP moldings producedby molding expanded, substantially non-crosslinked resin beads of a baseresin including a polypropylene resin (hereinafter referred to as“expanded PP beads”) in a mold is to use a high rigidity polypropyleneresin as a raw material. Thus, a propylene homopolymer or a propylenecopolymer containing a reduced content of a comonomer such as butene orethylene has been used. Such a high rigidity polypropylene resin,however, has a high melting point and requires a high temperature formolding. When steam is used for molding, it is necessary to use highpressure steam and, therefore, to use a special molding device having ahigh pressure resistance, in order to attain sufficient melt adhesionbetween the expanded PP beads.

[0005] Another known method for improving rigidity of PP moldings is touse expanded PP beads which show, in a DSC curve thereof, a hightemperature peak of a large area in addition to an intrinsic peak whichis present in a lower temperature side of the high temperature peak andis inherent to the polypropylene resin. In this case, too, it isnecessary to use high pressure steam and, therefore, to use a specialmolding device having a high pressure resistance, in order to attainsufficient melt adhesion between the expanded PP beads.

DISCLOSURE OF THE INVENTION

[0006] It is an object of the present invention to provide a processcapable of producing a PP molding having high rigidity (high compressionstrength) and high adhesion between beads by molding expanded PP beadsin a mold using steam at a relatively low temperature. Another object ofthe present invention is to provide a process capable of producing a PPmolding having excellent weatherability, heat resistance andrecyclability. It is a further object of the present invention toprovide an economical process for producing expanded PP beads.

[0007] In accomplishing the foregoing objects, there is provided inaccordance with one aspect of the present invention a process ofproducing a foamed molding, comprising the steps of:

[0008] (a) dispersing substantially non-crosslinked resin particles of abase resin including a polypropylene resin in a dispersing mediumcontaining an organic peroxide to obtain a dispersion;

[0009] (b) maintaining said dispersion at a temperature lower than themelting point of said base resin but sufficient to decompose saidorganic peroxide, thereby obtaining substantially non-crosslinked,surface-modified resin particles;

[0010] (c) foaming and expanding said non-crosslinked, surface-modifiedresin particles using a blowing agent to obtain expanded, substantiallynon-crosslinked resin beads;

[0011] (d) filling said expanded resin beads in a mold cavity;

[0012] (e) heating said expanded resin beads in said mold cavity tofuse-bond said expanded resin beads together into a unitary body; and

[0013] (f) cooling said unitary body.

[0014] In another aspect, the present invention provides a process ofproducing a foamed molding, comprising the steps of:

[0015] (a) filling expanded, substantially non-crosslinked resin beadsof a base resin including a polypropylene resin in a mold cavity, saidexpanded resin beads having a surface region and an inside regionsurrounded by said surface region, wherein each of said surface andinside regions shows a high temperature endothermic peak, in a DSC curvethereof, in addition to an intrinsic endothermic peak located at a lowertemperature side of said high temperature peak, and wherein said hightemperature endothermic peaks of said surface region and said insideregion have heat of fusion of Hs and Hi, respectively, and wherein Hsand Hi have the following relationship: Hs<0.86×Hi

[0016] (b) heating said expanded resin beads in said mold cavity tofuse-bond said expanded resin beads together into a unitary body; and

[0017] (c) cooling said unitary body.

[0018] The present invention also provides a process of producing afoamed molding, comprising the steps of:

[0019] (a) filling expanded, substantially non-crosslinked resin beadsof a base resin including a polypropylene resin in a mold cavity, saidexpanded resin beads exhibiting a high temperature endothermic peak, ina DSC curve thereof, in addition to an intrinsic endothermic peaklocated at a lower temperature side of said high temperature peak, eachof said expanded beads having a surface showing a melt initiationtemperature not higher than the melting point of said base resin,

[0020] (b) heating said expanded resin beads in said mold cavity tofuse-bond said expanded resin beads together into a unitary body; and

[0021] (c) cooling said unitary body.

[0022] The present invention further provides a process of producing afoamed molding, comprising the steps of:

[0023] (a) filling expanded, substantially non-crosslinked resin beadsof a base resin including a polypropylene resin in a mold cavity, saidexpanded resin beads exhibiting a high temperature endothermic peak, ina DSC curve thereof, in addition to an intrinsic endothermic peaklocated at a lower temperature side of said high temperature peak, eachof said expanded beads having a surface having an extrapolated meltinitiation temperature, as measured by micro differentialthermoanalysis, not higher than (Tm+4° C.) where Tm is the melting pointof the base resin,

[0024] (b) heating said expanded resin beads in said mold cavity tofuse-bond said expanded resin beads together into a unitary body; and

[0025] (c) cooling said unitary body.

[0026] The present invention further provides a process for thepreparation of expanded resin beads, comprising the steps of:

[0027] (a) dispersing substantially non-crosslinked resin particles of abase resin including a polypropylene resin in a dispersing mediumcontaining an organic peroxide to obtain a first dispersion having aweight ratio of said resin particles to said dispersing medium of 0.6:1to 1.3:1;

[0028] (b) maintaining said dispersion at a temperature lower than themelting point of said base resin but sufficient to decompose saidorganic peroxide, thereby obtaining substantially non-crosslinked,surface-modified resin particles dispersed in said dispersing medium;

[0029] (c) impregnating said surface-modified resin particles with ablowing agent, while maintaining the weight ratio of saidsurface-modified resin particles to said dispersing medium at 0.5 orless, to obtain a second dispersion; and

[0030] (d) reducing the pressure of said second dispersion to foam andexpand said surface-modified resin particles.

[0031] Other objects, features and advantages of the present inventionwill become apparent from the detailed description of the preferredembodiments of the invention which follows, when considered in light ofthe accompanying drawings, in which:

[0032]FIG. 1 is an initial DSC curve of expanded polypropylene beads;

[0033]FIG. 2 is a second time DSC curve of polypropylene resin particleswhich have not yet been subjected to surface modification and which havebeen once subjected to DSC measurement; and

[0034]FIG. 3 shows μDTA curves obtained by micro differentialthermoanalysis of surfaces of expanded PP beads obtained in Example 7and Comparative Example 5.

[0035]FIG. 4 is a schematic illustration of a molding device used inExample 9; and

[0036]FIG. 5 shows a μDTA curve obtained by micro differentialthermoanalysis of a surface of an expanded PP bead obtained in Example9.

[0037] The expanded PP beads according to the present invention areprepared by expanding substantially non-crosslinked resin particles of abase resin including a polypropylene resin. The term “polypropyleneresin” as used herein refers to (1) polypropylene homopolymer, (2) acopolymer of propylene and one or more comonomers having a propylenecontent of at least 60 mole %, a mixture of two or more of thecopolymers (2), or a mixture of the homopolymer (1) and the copolymer(2).

[0038] The copolymer may be, for example, ethylene-propylene blockcopolymers, ethylene-propylene random copolymers, propylene-butenerandom copolymers or ethylene-propylene-butene random copolymers.

[0039] The base resin containing the polypropylene resin as essentialcomponent preferably has a melting point of at least 130° C., morepreferably at least 135° C., still more preferably at least 145° C.,most preferably 158-170° C., for reasons of suitable physical propertiesof PP molding. For reasons of heat resistance of PP molding andexpansion efficiency in producing expanded particles, the base resinpreferably has a melt flow rate (MFR) of 0.3-100 g/10 min, morepreferably 1-90 g/10 min. The MFR herein is as measured in accordancewith the Japanese Industrial Standard JIS K7210-1976, Test Condition 14.

[0040] If desired, the base resin may contain one or more additionalresins or one or more elastomers. The amount of the additional resin orelastomer in the base resin is preferably no more than 35 parts byweight, more preferably no more than 25 parts by weight, still morepreferably no more than 15 parts by weight, most preferably no more than10 parts by weight, per 100 parts by weight of the polypropylene resin.Examples of the additional resins include polyethylene resins such ashigh density polyethylenes, medium density polyethylenes, low densitypolyethylenes, linear low density polyethylenes, linear very low densitypolyethylenes, ethylene-vinyl acetate copolymers, ethylene-acrylic acidcopolymers, ethylene-methacrylic copolymers; and polystyrene resins suchas polystyrene and styrene-maleic anhydride copolymers. Examples ofelastomers include ethylene-propylene rubber, ethylene-1-butene rubber,propylene-1-butene rubber, styrene-butadiene rubber, isoprene rubber,neoprene rubber, nitrile rubber, styrene-butadiene block copolymers andhydrogenated products of the above rubbers and copolymers.

[0041] The base resin may also be blended with one or more additivessuch as an antioxidant, a UV absorbing agent, a foam controlling agent,a sterically hindered amine compound, an antistatic agent, a fireretardant, a metal-deactivator, a pigment, a nucleus agent, a filler, astabilizer, a reinforcing material and a lubricant. The foam controllingagent may be, for example, an inorganic powder such as zinc borate,talc, calcium carbonate, borax or aluminum hydroxide.

[0042] The hindered amine compound (hereinafter referred to as HALS) isa compound containing the following structure (1):

[0043] It is preferred that HALS used have a molecular weight of400-10,000, more preferably 400-5,000, most preferably 2,500-5,000, forreasons of prevention of bleeding thereof on surfaces of the resinparticles and suitable dispersibility thereof in the resin particles.When HALS is a mixture of compounds having different molecular weights,the above “molecular weight” refers to an average molecular weight asmeasured by gel permeation chromatography using polystyrene as areference. As compared with known stabilizers for polypropylene resins,such as phenol compounds, phosphorus compounds and sulfur compounds,HALS exhibits superior heat resistance and weatherability.

[0044] Specific examples of HALS include dimethylsuccinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidinepolycondensate, bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate,poly[{6-(1,1,3,3-tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl){2,2,6,6-tetramethyl-4-piperidyl)imino}hexamethylene{2,2,6,6-tetramethyl-4-piperidyl)imino}],N,N′-bis(3-aminopropyl)ethylenediamine-2,4-bis[N-butyl-N-(1,2,2,6,6-pentamethyl-4-piperidyl)amino]-6-chloro-1,3,5-triazinecondensate, bis(1-octyloxy)-2,2,6,6-tetramethyl-4-piperidyl)sebacate,bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate,bis(1,2,6,6-pentamethyl-4-piperidyl)-2-(3,5-di-t-butyl-4-hydroxybenzyl)-2-n-butylmalonate. HALS is described in detail in SAKAI, Tsuruo, “Development andRecent Technology of Polymer Additive”, edited by CMC Co., Ltd.,published by Disc Co., Ltd., pages 9 and 16, May 29, 1992, thedisclosure of which is hereby incorporated by reference herein.

[0045] The amount of HALS is generally 0.01-2% by weight, preferably0.05-1% by weight, more preferably 0.05-0.5% by weight, based on theweight of the base resin, for reasons of improved heat resistance,weatherability and surface characteristics of the expanded PP beads.

[0046] The additive or additives are generally used in an amount of 20parts by weight or less, preferably 10 parts by weight or less, morepreferably 0.005-5 parts by weight, per 100 parts by weight of the baseresin.

[0047] The resin particles used as a raw material for the productionexpanded PP beads according to the present invention may be obtained byany suitable known method. For example, the above-described base resin,which is generally in the form of pellets, and, if desired, one or moreadditives are charged, mixed and kneaded in an extruder. The kneadedmass is then extruded through a die into strands and cut to obtain theresin particles. It is preferred that the strands be quenchedimmediately after being extruded for reasons that the succeeding surfacemodification with an organic peroxide, which will be describedhereinafter, may be efficiently performed. The quenching may be carriedout by introducing the strands in water at 50° C. or less, preferably40° C. or less, more preferably 30° C. or less. The cooled strands aretaken out of the water and cut into particles each having alength/diameter ratio of 0.5-2.0, preferably 0.8-1.3, and a mean weightof 0.1-20 mg, preferably 0.2-10 mg. The mean weight is an average of 200arbitrarily selected particles.

[0048] The resin particles are dispersed in a dispersing mediumcontaining an organic peroxide to obtain a dispersion. Any dispersingmedium may be used as long as it can disperse the resin particlestherein without substantially dissolving components of the particles.Examples of the dispersing medium include water, ethylene glycol,glycerin, methanol, ethanol or a mixture of them. An aqueous dispersionmedium, preferably water, more preferably ion-exchanged water, issuitably used.

[0049] The dispersion is heated at a temperature lower than the meltingpoint of the base resin but sufficient to decompose the organicperoxide, thereby obtaining substantially non-crosslinked,surface-modified resin particles. The non-crosslinked, surface-modifiedresin particles are then expanded using a blowing agent to obtainexpanded PP beads. The expanded PP beads have excellent fuse-bondingproperties and give a high rigidity PP molding in a mold using steam ata relatively low temperature.

[0050] Any organic peroxide may be used for the purpose of the presentinvention as long as it decomposes when heated at a temperature lowerthan the melting point of the base resin.

[0051] Illustrative of suitable organic peroxides are shown belowtogether with 1 Hr half life temperature Th and 1 min half lifetemperature Tn thereof, which are indicated in the brackets on the leftand right sides, respectively, of the slash (/) and which will bediscussed hereinafter:

[0052] Isobutylperoxide [50° C./85° C.],

[0053] Cumyl peroxy neodecanoate [55° C./94° C.],

[0054] α,α′-Bis (neodecanoylperoxy)diisopropylbenzene [54° C./82° C.],

[0055] di-n-Propyl peroxydicarbonate [58° C./94° C.],

[0056] Diisopropyl peroxydicarbonate [56° C./88° C.],

[0057] 1-Cyclohexyl-1-methylethyl peroxy neodecanoate [59° C./94° C.]

[0058] 1,1,3,3-Tetramethylbutyl peroxy neodecanoate [58° C./92° C.],

[0059] Bis (4-t-butylcyclohexyl) peroxydicarbonate [58° C./92° C.],

[0060] Di-2-ethoxyethyl peroxydicarbonate [59° C./92° C.],

[0061] Di (2-ethylhexylperoxy) dicarbonate [59° C./91° C.],

[0062] t-Hexyl peroxy neodecanoate [63° C./101° C.],

[0063] Dimethoxybutyl peroxydicarbonate [64° C./102° C.],

[0064] Di(3-methyl-3-methoxybutylperoxy)dicarbonate [65° C./103° C.],

[0065] t-Butyl peroxy neodecanoate [65° C./104° C.],

[0066] 2,4-Dichlorobenzoyl peroxide [74° C./119° C.],

[0067] t-Hexyl peroxy pivalate [71° C./109° C.],

[0068] t-Butyl peroxy pivalate [73° C./110° C.],

[0069] 3,5,5-Trimethylhexanoyl peroxide [77° C./113° C.],

[0070] Octanoyl peroxide [80° C./117° C.],

[0071] Lauroyl peroxide [80° C./116° C.],

[0072] Stearoyl peroxide [80° C./117° C.],

[0073] 1,1,3,3-Tetramethylbutyl peroxy 2-ethylhexanoate [84° C./124-C.],

[0074] Succinic peroxide [87° C./132° C.],

[0075] 2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane [83° C./119°C.],

[0076] 1-Cyclohexyl-1-methylethyl peroxy 2-ethylhexanoate [90° C./138°C.],

[0077] t-Hexyl peroxy 2-ethylhexanoate [90° C./133° C.],

[0078] t-Butyl peroxy 2-ethylhexanoate [92° C./134° C.],

[0079] m-Toluoyl benzoyl peroxide [92° C./131° C.],

[0080] Benzoyl peroxide [92° C./130° C.],

[0081] t-Butyl peroxy isobutylate [96° C./136° C.],

[0082] 1,1-Bis (t-butylperoxy)-2-methylcyclohexane [102° C./142° C.],

[0083] 1,1-Bis(t-hexylperoxy)-3,3,5-trimethylcyclohexane [106° C./147°C.],

[0084] 1,1-Bis(t-butylperoxy)-3,3,5-trimethylcyclohexane [109° C./149°C.],

[0085] 1,1-Bis (t-hexylperoxy)cyclohexane [107° C./149° C.],

[0086] 1,1-Bis (t-butylperoxy) cyclohexane [111° C./154° C.],

[0087] 2,2-Bis(4,4-di -butylperoxycyclohexyl)propane [114° C./154° C.],

[0088] 1,1-Bis (t-butylperoxy)cyclododecane [114° C./153° C.],

[0089] t-Hexyl peroxy isopropyl monocarbonate [115° C./155° C.],

[0090] t-Butyl peroxy maleic acid [119° C./168° C.],

[0091] t-Butyl peroxy 3,5,5-trimethylhexanoate [119° C./166° C.],

[0092] t-Butyl peroxy laurate [118° C./159° C.],

[0093] 2,5-Dimethyl-2,5-di (m-toluoylperoxy)hexane [117° C./156° C.],

[0094] t-Butyl peroxy isopropyl monocarbonate [118° C./159° C.],

[0095] t-Butyl peroxy 2-ethylhexyl monocarbonate [119° C./161° C.],

[0096] t-Hexyl peroxy benzoate [119° C./160° C.], and

[0097] 2,5-Dimethyl-2,5-di (benzoylperoxy)hexane [119° C./158° C.].

[0098] These organic peroxides may be used alone or in combination. Theamount of the organic peroxide in the dispersion is generally 0.01-10parts by weight, preferably 0.05-5 parts by weight, more preferably0.1-3 parts by weight, per 100 parts by weight of the resin particles.

[0099] In the dispersion obtained by dispersing the resin particles in adispersing medium containing an organic peroxide, it is preferred thatthe weight ratio of the resin particles to the dispersing medium be1.3:1 or less, more preferably 1.2:1 or less, much more preferably 1.1:1or less, most preferably 1:1 or less, for reasons of uniformly treatingthe particles with the organic peroxide. Namely, when the weight ratioof the resin particles to the dispersing medium is excessively high, adifficulty might be caused in uniformly treating the surfaces of theresin particles. Thus, a part of the resin particles which excessivelyundergo the surface modification tend to for an aggregate in thedispersion so that the discharge of the dispersion from the vessel atthe time of the expansion is not smoothly carried out. From thestandpoint of economy, the weight ratio of the resin particles to thedispersing medium is desirably at least 0.6:1, more preferably at least0.7:1.

[0100] An organic peroxide, when heated, decomposes and generatesradicals which causes three types of chain transfer reactions, namelyhydrogen extraction, addition and β-degradation. In the case of thepresent invention, the use of an organic peroxide capable of generatingradicals causing addition reactions, especially oxygen radicals, ispreferred. A carbonate-type organic peroxide is preferred for thisreason. The oxygen radicals may be organic oxy-radical (RO. where R isan organic group derived from the organic peroxide) as well as O-radical(O.). If desired, a chain transfer agent may be added to thepolypropylene resin particles-containing dispersion or previouslyincorporated into the resin particles.

[0101] Hitherto, the following methods are known to use an organicperoxide in connection with a polypropylene resin:

[0102] (1) A method in which polypropylene resin particles are uniformlyimpregnated with an organic peroxide and a crosslinking aid, theresulting resin particles being subsequently heated at a temperaturehigher than the melting point of the polypropylene resin to decomposethe organic peroxide and to crosslink the polypropylene resin;

[0103] (2) A method in which a composition containing polypropylene andan organic peroxide is kneaded in an extruder to decompose the organicperoxide and to decompose the polypropylene, thereby obtainingpolypropylene having a narrower molecular weight distribution(JP-A-H03-152136);

[0104] (3) A method in which polypropylene particles are uniformlyimpregnated with an organic peroxide and a crosslinking aid, theresulting resin particles being subsequently heated at a temperaturelower than the melting point of the polypropylene to decompose theorganic peroxide and to introduce a long chain branch or crosslinkingstructure into the polypropylene resin. The polypropylene resinparticles thus having an improved melt tension is kneaded with a blowingagent in an extruder and extruded (JP-A-H11-80262);

[0105] (4) A method in which a polypropylene resin is mixed and kneadedwith an organic peroxide and maleic anhydride in an extruder at atemperature higher than the melting point of the polypropylene resin tograft polymerize the maleic anhydride on the polypropylene resin.

[0106] The method of the present invention in which a dispersioncontaining the resin particles and an organic peroxide is maintained ata temperature lower than the melting point of the base resin butsufficient to decompose the organic peroxide, thereby obtainingsubstantially non-crosslinked, surface-modified resin particles is thusdistinct from the above known methods (1)-(4).

[0107] In the present invention, the organic peroxide is heated at atemperature lower than the melting point of the base resin butsufficient to substantially decompose the organic peroxide. It ispreferred that 1 Hr half life temperature Th (the temperature at whichthe amount of the organic peroxide decreases to half when the peroxideis heated at that temperature for 1 hour) of the organic peroxide be nothigher than the Vicat softening point of the base resin. The “Vicatsoftening point” in the present specification is in accordance withJapanese Industrial Standard JIS K 6747-1981. When the 1 Hr half lifetemperature Th is higher than the Vicat softening point of thepolypropylene resin, it is difficult to substantially decompose theorganic peroxide at a temperature lower than the melting point of thebase resin. When the decomposition of the organic peroxide is carriedout at a temperature not lower than the melting point of the base resin,the decomposed organic peroxide will attack not only the surfaces of theresin particles but also inside regions thereof, so that expanded PPbeads obtained cannot give a desired PP molding.

[0108] Thus, it is preferred that the 1 Hr half life temperature Th belower by at least 20° C., more preferably by at least 30° C., than theVicat softening point of the base resin. It is also preferred that the 1Hr half life temperature Th be in the range of 40-100° C., morepreferably 50-90° C., for reasons of easiness of handling.

[0109] The organic peroxide in the dispersion is desirably substantiallydecomposed at a temperature not higher than, more preferably lower by atleast 20° C. than, most preferably lower by at least 30° C. than, theVicat softening point .of the base resin. Further, the organic peroxidein the dispersion is desirably substantially decomposed at a temperaturenot lower than the glass transition point of the base resin, morepreferably at a temperature in the range of 40-100° C., most preferably50-90° C., for reasons of easiness in handling of the peroxide.

[0110] It is further preferred that the decomposition of the organicperoxide be performed by maintaining the organic peroxide at atemperature in the range of (Tn−30° C.) to (Tn+30° C.) for at least 10minutes, where Tn is 1 min half life temperature of the organic peroxide(the temperature at which the amount of the organic peroxide decreasesto half when the peroxide is heated at that temperature for 1 minute)for reasons of decomposition. efficiency. When the decomposition iscarried out at a temperature lower than (Tn−30° C.), a long time isrequired for completing the decomposition. Too high a decompositiontemperature in excess of (Tn+30° C.) might adversely affect theuniformity of surface treatment. From the standpoint of process cost andefficiency, the heat treatment at a temperature of (Tn−30° C.) to(Tn+30° C.) is desired to be performed for 60 minutes or shorter.Preferably, the dispersion of the resin particles in the organicperoxide-containing liquid medium is prepared at such a temperature thatthe peroxide is prevented from decomposing and, then, the temperature isincreased continuously or stepwise so that the peroxide is maintained ata temperature range of (Tn−30° C.) to (Tn+30° C.) for at least 10minutes. In this case, it is preferred that the peroxide be maintainedat a constant temperature of (Tn−5° C.) to (Tn+5° C.) for at least 5minutes.

[0111] The term “substantially decompose” as used herein means that theactive oxygen content of the peroxide is reduced to less than 50% of theoriginal value. Preferably, the peroxide is decomposed so that theactive oxygen content thereof be reduced to 30% or less, more preferably20% or less, most preferably 5% or less of the original value.

[0112] The “1 hour half life temperature Th” and “1 min half lifetemperature Tn” of the organic peroxide are measured as follows. Asample peroxide is dissolved in a suitable solvent inert to radicals,such as benzene or mineral spirit, to obtain a solution having aperoxide concentration of 0.1 mol/L or 0.05 mol/L. This is placed in aglass tube whose inside space has been substituted by nitrogen. Theglass tube is sealed and immersed in a constant temperature bathmaintained at a predetermined temperature for a given period (1 minuteor 1 hour) to permit the peroxide to decompose. The change inconcentration of the organic peroxide with the time is measured. Underthe above reaction conditions, since the decomposition reaction of theorganic peroxide can be regarded as being a first-order reaction, thefollowing equations can be formed:

dx/dt=k(a−x)

ln[a/(a−x)]=kt

[0113] wherein x denotes a concentration of the organic peroxide, adenotes the initial concentration of the organic peroxide, k denotes thedecomposition rate constant, and t denotes a time. Since the half-lifeperiod t_(1/2) is a time required for reducing the concentration of theorganic peroxide to half by decomposition (x=a/2), the followingrelationship is obtained:

kt_(1/2)=ln 2.

[0114] From the above measurement of the change in concentration of theorganic peroxide with the time (t), relationship between the time (t)and ln[a/(a−x)] is plotted to give a straight line. The gradientrepresents the constant (k). Thus, the half life t_(1/2) is calculatedfrom the above equation. The 1 Hr half life temperature and 1 min halflife temperature of an organic peroxide are the temperatures at whicht_(1/2) of the organic peroxide are 1 hour and 1 minute, respectively.

[0115] The “glass transition point” as used herein is measured inaccordance with JIS K7121-1987 and is calculated from the midpoint of aheat flux. The “glass transition point is measured after the sample hasbeen heat treated under specified conditions”.

[0116] In the present invention, the polypropylene resin, the baseresin, the resin particles, the surface-modified resin particles,expanded PP beads and PP molding are substantially non-crosslinked. Theterm “substantially non-crosslinked” as used herein is as defined below.Sample resin is immersed in xylene (100 ml xylene per 1 g sample resin)and the mixture is refluxed for 8 hours. The mixture is then immediatelyfiltered through a 74 μm wire net (specified in Japanese IndustrialStandard JIS Z8801-1966-). The dry weight of the xylene-insolublematters left on the wire net is measured. A crosslinking degree P (%) iscalculated from the formula:

P (%)=(M/L)×100

[0117] wherein M represents the weight (g) of the xylene-insolublematters and L represents the weight (g) of the sample. “Substantiallynon-crosslinked” means that the crosslinking degree P is 10% or less.

[0118] In the present invention, the crosslinking degree P of the baseresin, the resin particles, the surface-treated (or surface modified)resin particles, expanded PP beads and PP molding is preferably 5% orless, more preferably 3% or less, most preferably 1% or less. Ingeneral, the surface treatment does not result in an increase of thecrosslinking degree P.

[0119] The surface-modified resin particles are then foamed and expandedto obtain expanded PP beads using a blowing agent. Preferably, theexpansion step is carried out by a conventional dispersion method inwhich the resin particles are dispersed in a dispersing medium in aclosed vessel in the presence of a blowing agent and heated toimpregnate the resin particles with the blowing agent. While beingmaintained under a pressurized condition and at a temperature sufficientto expand the resin particles, the dispersion is discharged from thevessel to an atmosphere of a pressure lower than the pressure in thevessel, thereby obtaining expanded PP beads.

[0120] While the surface modification of the resin particles with theorganic peroxide and the subsequent expansion of the surface-modifiedresin particles may be carried out in separate vessels, it is preferredthat that the expansion step be carried out by the dispersion method andthat the expansion step be carried out in the same vessel for reasons ofefficiency. Namely, the surface modification the resin particles andexpansion of the surface-modified resin particles may be carried out bysimply conducting the dispersion method after addition of apredetermined amount of the organic peroxide in the dispersion.

[0121] In performing the expansion, it is preferred that the weightratio of the surface-modified resin particles to the dispersing mediumbe 0.5:1 or less, preferably 0.1:1 to 0.5:1, for reasons of preventionof melt adhesion of the surface-modified resin particles in thedispersion. Thus, when the surface modification of the resin particlesis carried out in a vessel with the ratio of the resin particles to thedispersing medium being maintained in a range of 0.6:1 to 1.3:1, andwhen the expansion is performed in the same vessel, a fresh dispersingmedium is added to the vessel before subjecting the dispersion to theexpansion step.

[0122] The surface-modified resin particles, expanded PP beads obtainedtherefrom and PP molding obtained from the beads may contain 100-8000ppm by weight of an alcohol having a molecular weight of 50 or more andproduced by the decomposition of the organic peroxide. For example,p-t-butylcyclohexanol may be present in the expanded PP beads, whenbis(4-t-butylcyclohexyl)peroxydicarbonate is used as the organicperoxide. i-Propanol, s-butanol, 3-methoxybutanol, 2-ethylhexylbutanolor t-butanol may be detected, when the corresponding peroxide is used.

[0123] To prevent melt-adhesion of the surface-treated resin particleswith each other during the expansion step, it is desirable to add to thedispersing medium a dispersing agent which is finely divided organic orinorganic solids. For reasons of easiness of handling, the use of aninorganic powder is preferred. Illustrative of suitable dispersingagents are natural or synthetic clay minerals (such as kaolin, mica,pyrope and clay), alumina, titania, basic magnesium carbonate, basiczinc carbonate, calcium carbonate and iron oxide. The dispersing agentis generally used in an amount of 0.001-5 parts by weight per 100 partsby weight of the resin particles.

[0124] To improve the dispersing efficiency of the dispersing agent,namely to reduce the amount of the dispersing agent while retaining itsfunction to prevent melt-adhesion of the surface-treated particles, adispersion enhancing agent may be preferably added to the dispersingmedium. The dispersion enhancing agent is an inorganic compound capableof being dissolved in water in an amount of at least 1 mg in 100 ml ofwater at 40° C. and of providing divalent or trivalent anion or cation.Examples of the dispersion enhancing agents include magnesium chloride,magnesium nitrate, magnesium sulfate, aluminum chloride, aluminumnitrate, aluminum sulfate, ferric chloride, ferric sulfate and ferricnitrate. The dispersion enhancing agent is generally used in an amountof 0.0001-1 part by weight per 100 parts by weight of the polypropyleneresin particles.

[0125] The blowing agent may be an organic physical blowing agent or aninorganic physical blowing agent. Examples of the organic physicalblowing agents include aliphatic hydrocarbons such as propane, butane,pentane, hexane and heptane, alicyclic hydrocarbons such as cyclobutaneand cyclohexane, and halogenated hydrocarbons such aschlorofluoromethane, trifluoromethane, 1,2-difluoroethane,1,2,2,2-tetrafluoroethane, methylchloride, ethylchloride andmethylenechloride. Examples of inorganic physical blowing agents includeair, nitrogen, carbon dioxide, oxygen, argon and water. These organicand inorganic blowing agents may be used singly or as a mixture of twoor more. For reasons of stability (uniformity) of apparent density ofexpanded PP beads, low costs and freedom of environmental problem, theuse of air or nitrogen is preferred. Water as the blowing agent may bethat used in dispersing the surface-modified resin particles in thedispersing medium.

[0126] The amount of the blowing agent may be suitably determinedaccording to the kind of the blowing agent, expansion temperature andapparent density of the expanded PP beads to be produced. When nitrogenis used as the blowing agent and when water is used as the dispersingmedium, for example, the amount of nitrogen is preferably such that thepressure within the closed vessel in a stable state immediately beforethe initiation of the expansion, namely the pressure (gauge pressure) inthe upper space in the closed vessel, is in the range of 0.6-8 MPa(G).In general, the pressure in the upper space in the closed vessel isdesirably increased as the apparent density of the expanded PP beads tobe obtained is reduced.

[0127] It is preferred that the expansion of the surface-modified resinparticles be performed so that the expanded PP beads have an apparentdensity of 10 g/L to 500 g/L. The apparent density (g/L) is obtained bydividing the weight W (g) of the expanded PP beads by the volume V (L)of the apparent volume thereof (density=W/V). The apparent volume ismeasured as follows:

[0128] In a measuring cylinder, about 5 g of expanded PP beads areallowed to stand at 23° C. for 48 hours in the atmosphere and thereafterimmersed in 100 ml water contained in a graduation cylinder at 23° C.From the increment of the volume, the apparent volume can be determined.

[0129] It is preferred that the expansion of the surface-modified resinparticles be performed so that the expanded PP beads have a hightemperature endothermic peak, in a DSC curve thereof, in addition to anintrinsic endothermic peak located at a lower temperature side of thehigh temperature peak, because the expanded PP beads have high contentof closed cells and extremely suited to obtain a high rigidity PPmolding.

[0130] The high temperature peak preferably has such an areacorresponding to heat of fusion (calorific value; absolute value) in therange of 2-70 J/g, more preferably 3-65 J/g, most preferably 12-58 J/g.When the heat of fusion of the high temperature peak is less than 2 J/g,the compression strength and shock absorbing power of a PP molding tendto be reduced. Too high a heat of fusion of the high temperature peak inexcess of 70 J/g requires a high pressure to increase the insidepressure in the beads before the molding step. It is preferred that theheat of fusion of the high temperature peak is 10-60%, more preferably20-50%, of a total of the heat of fusion of the high temperature peakand the heat of fusion of the intrinsic peak. The total heat of fusionis suitably in the range of 40-150 J/g.

[0131] The DSC curve herein is as obtained by the differential scanningcalorimetric analysis wherein a sample (2-4 mg of expanded PP beads) isheated from room temperature (10-40° C.) to 220° C. in an atmosphere ofnitrogen at a rate of 10° C./min. FIG. 1 shows an example of a DSC curvehaving an intrinsic endothermic peak P1 at a peak temperature T1 and ahigh temperature endothermic peak P2 at a peak temperature T2. The areaof a peak corresponds to the heat of fusion thereof.

[0132] The area of the high temperature peak P2 is determined asfollows. In the DSC curve (first DSC curve) C having two endothermicpeaks P1 and P2 at temperatures T1 and T2, respectively, as shown inFIG. 1, a straight line A extending between the point Z1 in the curve at80° C. and the point Z2 in the curve at a melt completion temperatureTmc is drawn. The melt completion temperature Tmc is represented by apoint at which the high temperature peak P2 ends and meets the base lineon a high temperature side. Next, a line B which is parallel with theordinate and which passes a point B_(C) between the peaks P1 and P2 isdrawn. The line B crosses the line A at a point B_(A). The position ofthe point B_(C) is such that the length between the point B_(A) and thepoint B_(C) is minimum. The area of the high temperature peak P2 is theshaded area defined by the line A, line B and the DSC curve C. A totalof the heat of fusion of the high temperature peak P2 and the heat offusion of the intrinsic peak P1 corresponds to an area defined by theline A and the DSC curve.

[0133] When expanded PP beads having a weight per bead of less than 2 mgare measured for the intrinsic peak P1 and high temperature peak P2using a differential scanning calorimeter, two or more beads are sampledfor the measurement such that the total weight of the sample is in therange of 2-10 mg. When expanded PP beads to be measured have a weightper bead of 2-10 mg, one bead is sampled for the DSC measurement. Whenexpanded PP beads to be measured have a weight per bead of more than 10mg, one of the beads is cut into two or more pieces and one of thepieces having a weight of 2-10 mg is sampled for the DSC measurement. Inthis case, an expanded PP bead having a weight W and an outer peripheralsurface area of S is preferably cut into n number of pieces so that cutpieces have nearly equal weight of W/n and have a surface portion whichis derived from the outer peripheral surface of the bead and which hasan area of nearly S/n. For example, when the expanded PP beads to bemeasured have a weight per bead of 18 mg, one of the beads is cut alonga plane bisecting the bead and one of the cut pieces is used formeasurement. In the present specification, except otherwise noted, theterm “heat of fusion of the high temperature peak of expanded PPbead(s)” is intended to refer to the heat of fusion as measured in theabove-described method, and should be discriminated from “heat of fusionof the high temperature peak of a surface region or an inside region ofan expanded PP bead” which will be described hereinafter.

[0134] The above-described high temperature peak P2 is present in theDSC curve measured first. Once the expanded PP beads have completelymelted, the high temperature peak P2 no longer appears. Thus, when thesample after the first DSC measurement is cooled to room temperature(10-40° C.) and is measured again for a DSC curve by heating to 220° C.in an atmosphere of nitrogen at a rate of 10° C./min, the second DSCcurve does not show such a high temperature peak but contains anendothermic peak attributed to the melting of the base resin, just likea DSC curve shown in FIG. 2.

[0135] In the present specification and claims, the term “melting pointof the base resin” is intended to refer to that measured by DSC analysisof base resin particles which have not yet been subjected to surfacemodification treatment with an organic peroxide. Namely, “melting pointof the base resin” is measured by the differential scanning calorimetricanalysis wherein a sample (2-4 mg of resin particles of the base resin)is heated from room temperature (10-40° C.) to 220° C. in an atmosphereof nitrogen at a rate of 10° C./min. The sample is then cooled to roomtemperature (10-40° C.) and is measured again for a DSC curve by heatingto 220° C. in an atmosphere of nitrogen at a rate of 10° C./min toobtain a second DSC curve as shown in FIG. 2. The temperature Tm of theendothermic peak P3 at 130-170° C. in the second DSC curve as shown inFIG. 2 is inherent to the polypropylene resin and represents the“melting point of the base resin”. Two or more endothermic peaks mightbe observed in the second DSC curve, when, for example, the resinparticles are composed of two or more different polypropylene resins. Inthis case, the melting point Tm is the peak temperature of that peakwhich has the greatest peak height among those peaks. When there are aplurality of peaks having the same greatest peak height, then themelting point Tm is the highest peak temperature among those peaks. Theterm “peak height” herein refers to the length S between the top of thepeak P3 and a point Q at which a line parallel with the ordinate andpassing through the top of the peak P3 crosses the base line B_(L). InFIG. 2, the temperature Te at which the endothermic peak P3 ends andmeets the base line B_(L) refers to the “melt completion temperature ofthe base resin”.

[0136] The high temperature peak P2 of expanded PP beads generallyappears at a temperature T2 ranging from (Tm+5° C.) to (Tm+15° C.). Theendothermic peak P1 of expanded PP beads generally appears at atemperature T1 ranging from (Tm−5° C.) to (Tm+5° C.). The endothermicpeak in the second DSC measurement of expanded PP beads generallycorresponds to that in the second DSC curve of the precursor base resinparticles and generally appears at a temperature ranging from (Tm−2° C.)to (Tm+2° C.).

[0137] As described above, it is preferred that the expanded PP beadshave such a crystal structure that a high temperature peak is present ina first DSC curve thereof in addition to an intrinsic peak. A differencebetween the melting point of the polypropylene resin and expansiontemperature has a great influence upon the heat of fusion (peak area) ofthe high temperature peak.

[0138] The heat of fusion of the high temperature peak of the expandedPP beads is a factor for determining the minimum temperature of steamwhich provides a saturated steam pressure required for melt-bonding thebeads to each other. In general, when the same base resin is used, thesmaller the heat of fusion of the high temperature peak, the lowerbecomes the minimum temperature. Further, the higher the expansiontemperature, the smaller becomes the heat of fusion of the hightemperature peak.

[0139] When expanded PP beads having a small heat of fusion of the hightemperature peak are used, the mechanical properties of the resulting PPmolding are relatively low, though the minimum temperature required formelt-bonding the beads can be low. On the other hand, when expanded PPbeads having a large heat of fusion of the high temperature peak areused, the mechanical properties of the resulting PP molding arerelatively high. In this case, however, since the minimum temperaturerequired for melt-bonding the beads is high, it is necessary to use highpressure steam for the production of PP moldings. Thus, the mostpreferred expanded PP beads would be such that the heat of fusion of thehigh temperature peak thereof is large but the minimum temperaturerequired for melt-bonding the beads is low. The present invention doesprovide such ideal expanded PP beads. The expanded PP beads according tothe present invention can give a high rigidity PP molding without usinga high temperature steam.

[0140] The expanded PP beads providing a DSC curve having such a hightemperature peak can be suitably produced by maintaining the dispersioncontaining the surface-modified resin particles in a vessel at a firstfixed temperature between a temperature lower by 20° C. than the meltingpoint of the base resin (Tm−20° C.) and a temperature lower than themelt completion point of the base resin (Te) for a period of time ofpreferably 10-60 min, preferably 15-60 min and then discharging thedispersion from the vessel after increasing the temperature of thedispersion to a second fixed temperature between a temperature lower by15° C. than the melting point of the base resin (Tm−15° C.) and atemperature higher by 10° C. than the melt completion point of the baseresin (Te+10° C.) or, if necessary, after maintaining the dispersion atthe second fixed temperature for a period of time of 10-60 min.

[0141] The area of the high temperature peak mainly depends upon theabove first fixed temperature at which the dispersion is maintainedbefore expansion treatment, the time for which the dispersion ismaintained at the first fixed temperature, the above second fixedtemperature, the time for which the dispersion is maintained at thesecond fixed temperature, the heating rate at which the dispersion isheated to the first fixed temperature and the heating rate at which thedispersion is heated from the first fixed temperature to the secondfixed temperature. The area of the high temperature peak increases withan increase of the retention time at the first and second fixedtemperatures. The heating rate (average heating rate from thecommencement of heating until the fixed temperature is reached) in eachof the heating stage up to the first fixed temperature and thesucceeding heating stage from the first fixed temperature to the secondfixed temperature is generally 0.5-5° C. per minute. Suitable conditionsfor the preparation of expanded PP beads having desired heat of fusionof the high temperature peak can be determined by preliminaryexperiments on the basis of the above points.

[0142] The above temperature ranges for the formation of the hightemperature peak and for the expansion of the resin particles aresuitably adopted in the case where an inorganic physical blowing agentis used. When an organic physical blowing agent is used, the suitabletemperature ranges will shift toward low temperature side and vary withthe kind and amount of the organic physical blowing agent.

[0143] The expanded PP beads used for the production of a foamed moldingaccording to the present invention preferably have at least one of thefollowing characteristics.

[0144] A surface region of the expanded PP bead preferably has a meltingpoint (Tms) lower than the melting point (Tmi) of an inside regionthereof (Tms<Tmi). The difference between the melting point (Tmi−Tms) ispreferably at least 0.05° C., more preferably at least 0.1° C., mostpreferably at least 0.3° C. The melting point Tms is determined asfollows. A surface region of the expanded PP bead is cut and about 2-4mg of such cut samples are collected. The sample is subjected to DSCanalysis in the same manner as described previously with regard to themeasurement of the melting point Tm. The peak temperature of a peakcorresponding to the endothermic peak P3 in the second DSC curverepresents the melting point Tms. The melting point Tmi is also measuredin the same manner as above except that inside region of the bead is cutand collected.

[0145] In the case of the expanded PP bead having a high temperatureendothermic peak in a DSC curve thereof, the heat of fusion Hs of thehigh temperature endothermic peak of the surface region of the bead ispreferably smaller than the heat of fusion Hi of the high temperatureendothermic peak of the inside region of the bead such that thefollowing relationship is established:

Hs<0.86×Hi

[0146] for reasons that the expanded PP beads can be molded at a lowertemperature as compared with surface unmodified expanded PP beads. Suchan effect increases with a decrease of Hs. Thus, the Hs and Hi of theexpanded PP bead preferably have the following relationship:

Hs<0.83×Hi, more preferably Hs<0.80×Hi,

still more preferably Hs<0.75×Hi,

yet still more preferably Hs<0.70×Hi,

most preferably Hs<0.60×Hi.

[0147] Preferably, Hs is not smaller than 0.25×Hi (Hs>0.25×Hi).

[0148] It is also preferred that Hs is in the range of 1.7-60 J/g, morepreferably 2-50 J/g, still more preferably 3-45 J/g, most preferably4-40 J/g, for reasons of availability of a low molding temperature

[0149] The surface region and inside region of an expanded PP bead aresampled by cutting the bead with a knife or a microtome. The surfaceregion or regions are sliced off the bead at any arbitral position orpositions to a thickness of 200 μm or less such that the outer surfaceof the bead provides one of the both sides of each of the sliced surfaceregions. Thus, the other side of each of the sliced surface regions doesnot contain that part of the PP bead which was present at a depth ofmore than 200 μm before cutting. The depth herein is in the directionfrom the outer surface of the bead to the center of gravity thereof.When the sliced surface region or regions contain that part of the PPbead which was present at a depth of more than 200 μm, precise datacannot be obtained. When the amount of the surface region or regionssampled from the bead is less than 2 mg, one or more additional beadsare cut to collect 2-4 mg of the sample.

[0150] The inside region is obtained by removing all of the surfaceregion of the bead up to the depth of 200 μm in the direction from theouter surface of the bead to the center of gravity thereof. When thesize of the bead is so small that no inside region is obtainable afterremoval of surface region of the 200 μm thick, then the inside region isobtained by removing all of the surface region of the bead up to thedepth of 100 μm in the direction from the outer surface of the bead tothe center of gravity thereof. When the size of the bead is so smallthat no inside region is obtainable after removal of surface region ofthe 100 μm thick, then the inside region is obtained by removing all ofthe surface region of the bead up to the depth of 50 μm in the directionfrom the outer surface of the bead to the center of gravity thereof.When the amount of the inside region obtained from one bead is less than2 mg, one or more additional beads are used to collect 2-4 mg of thesample. The thus collected samples are measured for the melting pointand heat of fusion of the high temperature peak according to the methoddescribed above.

[0151] The expanded PP bead preferably has a surface having a meltinitiation temperature, as measured by micro differentialthermoanalysis, not higher than the melting point of the base resin. Inthe conventional expanded PP beads, the melt initiation temperature ishigher by at least 5° C. than the melting point of the base resin.

[0152] Further, the expanded PP bead preferably has a surface having anextrapolated melt initiation temperature, as measured by microdifferential thermoanalysis, not higher than (Tm+4° C.) where Tm is themelting point of the base resin. In the conventional expanded PP beads,the extrapolated melt initiation temperature is higher by at least 8° C.than the melting point (Tm).

[0153] The micro differential thermoanalysis (μDTA) is performed using amicro differential thermoanalysis system (“Type 2990 Micro ThermalAnalyzer” of T. A. Instrument, Japan Inc.) at a heating rate of 10°C./sec from 25° C. to 200° C. The “melt initiation temperature” as usedherein is intended to refer to a temperature at which a μDTA curvestarts separating from the base line thereof. The “extrapolated meltinitiation temperature” as used herein is intended to refer to atemperature at the intersection of the base line and a tangential linedrawn from such a point on the μDTA curve on the higher temperature sideof the melt initiation temperature that the gradient of the tangentialline relative to the base line is maximum. For example, in the lowerμDTA curve Cm shown in FIG. 3, Pm and Pme represent the melt initiationtemperature (about 131° C.) and extrapolated melt initiation temperature(about 135° C.), respectively. At Pme, a tangentially extrapolated lineTL extending from a point Km on the curve Cm intersects the base lineBL. The point Km is so located on the higher temperature side of themelt initiation temperature Pm as to provide the maximum gradient of thetangential line TL relative to the base line BL. Similarly, in the upperμDTA curve Cnm, Pnm and Pnme represent the melt initiation temperature(about 168° C.) and extrapolated melt initiation temperature (about 171°C.), respectively. In FIG. 5, Pm and Pme represent the melt initiationtemperature (about 140° C.) and extrapolated melt initiation temperature(about 142° C.), respectively.

[0154] The μDTA is performed by fixing a sample expanded PP bead on asample stage. When the bead has an excessively large size, the bead maybe cut into a suitable size. A probe tip of the thermal analyzer isdirected toward an arbitral region of the surface of the bead and thendisplaced to contact with the surface of the bead. Then, the measurementis carried out while maintaining the contact state. The probe tip has asize of 0.2 μm×0.2 μm. Similar measurement is repeated at 10 differentpositions on a surface of the bead to obtain 10 μDTA curves in total,from each of which the melt initiation temperature and extrapolated meltinitiation temperature are determined. The “melt initiation temperature”and “extrapolated melt initiation temperature” as used herein are eachan arithmetic mean of the eight values remaining after omitting themaximum and minimum values from the ten measured values. When two ormore maximum and/or two or more minimum values exist, an arithmetic meanis calculated from the values remaining after omitting the maximum andminimum values. When all of the ten measured values are the same, thenthat value represents the arithmetic mean. When there are no othervalues than the maximum and minimum values and when the differencebetween them is not greater than 10° C., then an arithmetic mean of theten values is adopted. When there are no other values than the maximumand minimum values and when the difference between them is greater than10° C., then μDTA is repeated for another ten different points until theabove-defined “melt initiation temperature” and “extrapolated meltinitiation temperature” are determined.

[0155] The reduction of the melt initiation temperature and/or meltinitiation temperature in the expanded PP bead of the present inventionis considered to contribute to a reduction of the minimum temperaturerequired for melt-bonding the beads. The melt initiation temperature ispreferably Tm or less, more preferably (Tm−5° C.) or less, even morepreferably (Tm−10° C.) or less, still more preferably (Tm−15° C.) orless, yet still more preferably (Tm−50° C.) to (Tm−16° C.), mostpreferably (Tm−35° C.) to (Tm−17° C.). The extrapolated melt initiationtemperature is preferably (Tm−1° C.) or less, more preferably (Tm−6° C.)or less, even more preferably (Tm−11° C.) or less, still more preferably(Tm−16° C.) or less, most preferably (Tm−35° C.) to (Tm−17° C.).

[0156] Such a reduction of the minimum temperature required formelt-bonding the beads is particularly advantageous when the base resinof the expanded PP beads has a melting point of 158° C. or more and whenthe expanded PP beads has a high temperature peak. When the base resinof the expanded PP beads has a melting point of 158° C. or more, it ispreferred that the melt initiation temperature be 158° C. or less, morepreferably 155° C. or less, still more preferably 150° C. or less, yetstill more preferably 110-147° C., most preferably 125-145° C. While thelower the melt initiation temperature of the surfaces of the expanded PPbeads is, the lower is the minimum temperature required for melt-bondingthe beads, an excessively low melt initiation temperature will cause areduction of mechanical strength, such as compression strength, of a PPmolding obtained from the expanded PP beads.

[0157] The expanded PP bead preferably has an MFR value which is notsmaller than that of the resin particles before the surface modificationwith the organic peroxide and which is in the range of 0.5-150 g/10 min,more preferably 1-100 g/10 min, most preferably 10-80 g/10 min. It isalso preferred that the MFR value of the expanded PP bead be at least1.2 times, more preferably at least 1.5 times, most preferably 1.8-3.5times, that of the resin particles prior to the surface modification.

[0158] For measuring the MFR, the expanded PP beads are pressed at 200°C. using a heat press into a sheet having a thickness of 0.1-1 mm.Pellets or columns are prepared from the sheet to obtain a sample. Thesample is measured for MFR in accordance with the Japanese IndustrialStandard JIS K7210-1976, Test Condition 14. In the measurement of MFR,air bubbles must be removed from the sample. If necessary, heat presstreatment should be repeated up to three times in total to obtainbubble-free sheet.

[0159] The expanded PP bead preferably has a surface region having agreater oxygen content per unit weight than that of the inside region.When the organic peroxide used for the surface modification of the resinparticles is of a type which generates oxygen radicals upon beingdecomposed, part of the oxygen radicals are bound to surfaces of theparticles. The analysis, using an infrared spectrometer equipped withthe attenuated total reflectance (ATR analysis), of a surface of a PPmolding obtained from expanded PP beads of the present invention shows astronger absorption at a wavelength of near 1033 cm⁻¹ than that of a PPmolding obtained from conventional expanded PP beads. Thus, the ratio ofthe peak height at 1033 cm⁻¹ to the peak height at 1166 cm⁻¹ in the caseof the PP molding of the present invention is greater than that of theconventional molding. Further, the analysis using an energy dispersionspectroscope (EDS) shows that a surface of the expanded PP beadaccording to the present invention has an oxygen to carbon molar ratio(O/C molar ratio) is 0.2 whereas an inside of the bead has an O/C molarratio of 0.1. Further, a surface of the conventional expanded PP beadhas O/C molar ratio of 0.1. The preferred O/C ratio is at least 0.15.

[0160] Although not wishing to be bound by the theory, such anoxygen-added surface of the expanded PP bead is considered to enhancesteam permeability thereof. As a result of one of the foregoingcharacteristics (namely, Tms<Tmi; Hs<0.86×Hi; melt initiationtemperature≦melting point; extrapolated melt initiationtemperature≦melting point+4° C.; and oxygen-added surface) or as aresult of synergetic effect of two or more of the foregoingcharacteristics, the minimum temperature required for melt-bonding thebeads is lowered while ensuring high mechanical strengths of a PPmolding obtained from the beads.

[0161] The expanded PP beads obtained by the above process are aged inthe atmosphere. If desired, the PP beads may be treated to increase thepressure inside of the cells thereof and, thereafter, heated with steamor hot air to improve the expansion ratio thereof.

[0162] A PP molding may be suitably obtained by a batch-type moldingmethod in which expanded PP beads (if necessary, after being treated toincrease the pressure inside of the cells thereof) are filled in a moldadapted to be heated and cooled and to be opened and closed. Afterclosing the mold, saturated steam is fed to the mold to heat andfuse-bond the beads together. The mold is then cooled and opened to takea PP molding-out of the mold. A number of molding machines arecommercially available. They are generally designed to have a pressureresistance of 0.41 MPa(G) or 0.45 MPa(G). Thus, the above method isgenerally carried out using steam having a pressure of 0.45 MPa(G) orless, more preferably 0.41 MPa(G) or less.

[0163] A PP molding may be also produced by a continuous method in whichexpanded PP beads (if necessary, after being treated to increase thepressure inside of the cells thereof) are fed to a path which is definedbetween a pair of belts continuously running in the same direction andwhich has a heating zone and a cooling zone. During the passage throughthe heating zone, the expanded PP beads are heated with saturated steamand fuse-bonded to each other. The resulting molding is cooled in thecooling zone, discharged from the path and cut to a desired length. Theabove continuous method is disclosed in, for example, JP-A-H09-104026,JP-A-H09-104027 and JP-A-H10-180888.

[0164] The above-mentioned treatment of the expanded PP beads toincrease the pressure inside of the cells thereof may be carried out byallowing the beads to stand for a suitable period of time in a closedvessel to which a pressurized gas has been fed. Any gas containing aninorganic gas as a major ingredient may be used for the pressureincreasing treatment as long as it is in the form of gas underconditions where the expanded beads are treated. Examples of theinorganic gas include nitrogen, oxygen, air, carbon dioxide and argon.Nitrogen or air is suitably used for reasons of costs and freedom ofenvironmental problems.

[0165] Described below will be a specific method of increasing theinside pressure of the cells using air and a method of measuring thethus increased inside pressure in the cells.

[0166] Expanded PP beads are placed in a closed vessel into whichpressurized air is fed. The beads are allowed to stand in the vessel fora certain period of time (generally several hours) while maintaining thepressure inside the vessel at 0.98-9.8 MPaG so that the inside pressureof the cells increases. The thus treated expanded PP beads are placed ina mold for the production of a PP foam molding. The inside pressure ofthe cells Pi (MPa(G)) as used herein is defined as follows:

Pi=Wi×R×Te/(M×V)

[0167] wherein

[0168] Wi is an amount of air increased (g),

[0169] R is the gas constant and is 0.0083 (MPa·L/(K·mol),

[0170] Te is an ambient temperature and is 296K,

[0171] M is the molecular weight of air and is 28.8 (g/mol), and

[0172] V is the volume (liter) of the air in the expanded beads.

[0173] The amount of air increased Wi (g) is measured as follows.

[0174] A quantity of expanded beads whose cells have been justpressurized with air in the vessel are taken out of the vessel andcollected in a polyethylene film bag having a size of 70 mm×100 andprovided with a multiplicity of perforations each having a sizepreventing the passage of the beads. The beads in the bag are placed,within 60 seconds after the take-out, on a weighing device provided in athermostatic chamber maintained at 23° C. and 50% relative humidityunder ambient pressure. The weight Ua (g) of the beads is measured just120 seconds after the expanded beads have been taken out from thevessel. The expanded beads are then allowed to stand for 48 hours in thechamber at 23° C. and 50% relative humidity under ambient pressure. Theair in the cells of the expanded beads gradually permeates through thecell walls and escapes from the beads. Therefore, the weight of thebeads decreases with the lapse of time. However, an equilibrium has beenestablished and the weight decrease no longer occurs after lapse of the48 hours period. Thus, the weight of the expanded beads Ub (g) ismeasured in the same chamber after the lapse of the 48 hours period. Ofcourse, the weight of the polyethylene bag is also measured and taken inconsideration. The measurement of the weight should be carried outprecisely to the fourth decimal place (0.0001 g). The balance betweenthe weights Ua and Ub represents the amount of gas increased (Wi=Ua−Ub).

[0175] The volume of the air in the expanded PP beads V (L) is definedas follows.

V(L)=Va−Vb

[0176] wherein

[0177] Va is the apparent volume of the expanded PP beads, and

[0178] Vb is the volume of the base resin of the beads and is obtainedby dividing the weight of the beads Ub (g) by the density of the baseresin (g/L).

[0179] The apparent volume Va (L) of the expanded PP beads is measuredas follows. The expanded PP beads which have been subjected to themeasurement of the weight Ub as described above, are immersed in 100 mlof water at 23° C. contained in a graduated measuring cylinder. From thevolume increment, apparent volume Va (L) of the beads is determined. Thequantity of the above-described expanded beads sampled and collected inthe bag is such that Ub and Va fall within the ranges of 0.5 to 10 g and50 to 90 cm³ respectively.

[0180] The inside pressure Pi of the cells of the expanded PP beads ispreferably 0.98 MPa(G) or less, more preferably 0.69 MPa(G) or less,still more preferably 0.69 MPa(G) or less. The apparent density of thePP molding obtained by the above methods may be controlled as desiredand is generally in the range of 9-600 g/L. The term “apparent density”of the PP molding as used herein is as specified in JIS K7222-1999. Thevolume of a PP molding used for the calculation of the apparent densityis determined from the external dimensions thereof. When the externalshape of the molding is so complicated that the volume thereof isdifficult to be determined, then the volume thereof is measured byimmersing the molding in water and is given as a volume of waterreplaced by the molding. The PP molding preferably has open cell content(according to ASTM-D2856-70, Procedure C) of 40% or less, morepreferably 30% or less, most preferably 25% or less, for reasons of highmechanical strengths.

[0181] A surface layer, such as a reinforcing layer or a decorativelayer) may be integrally provided on a surface of the above PP molding.A method of producing such a composite article is disclosed in, forexample, U.S. Pat. No. 5,928,776, U.S. Pat. No. 6,096,417, U.S. Pat. No.6,033,770, U.S. Pat. No. 5,474,841, EP-B-477476, WO98/34770, WO98/00287and JP-B-3092227, the disclosure of which is hereby incorporated byreference herein.

[0182] An insert may be integrated with the above PP molding such thatat least part of the insert is embedded therein. A method of producingsuch a composite article is disclosed in, for example, U.S. Pat. No.6,033,770, U.S. Pat. No. 5,474,841, JP-A-S59-1277714 and JP-B-3092227,the disclosure of which is hereby incorporated by reference herein.

[0183] The following examples will further illustrate the presentinvention. Parts are by weight.

EXAMPLES 1-7 and COMPARATIVE EXAMPLES 1-5

[0184] 100 Parts of polypropylene resin selected from those shown inTable 1 and indicated in Tables 3-1 and 3-2 were blended with 0.05 partof zinc borate powder (cell controlling agent) and the blend was kneadedin an extruder and extruded into strands. The strands were immediatelyintroduced in water at 18° C. for quenching. The cooled strands werethen cut into particles each having a length/diameter ratio of about 1.0and a mean weight of 2 mg.

[0185] In a 400 liter autoclave, 100 parts of the above resin particlesare charged together with 220-parts of ion-exchanged water at 18° C.,0.005 part of sodium dodecylbenzenesulfonate (surfactant), 0.3 part ofkaolin powder (dispersing agent), 0.01 part of aluminum sulfate powder(dispersion enhancing agent), an organic peroxide selected from thoseshown in Table 2 and indicated in Table 3-1 or 3-2 in an amount shown inTable 3-1 or Table 3-2, and carbon dioxide (blowing agent) in an amountshown in Table 3-1 or 3-2. The mixture in the autoclave was dispersedwith stirring and heated to a temperature lower by 5° C. than theexpansion temperature shown in Table 3-1 or 3-2 at an average heatingrate of 3° C./min and then maintained at that temperature for 15 min.Thereafter, the temperature was raised to the expansion temperature atan average heating rate of 3° C./min and maintained at that temperaturefor 15 min. One end of the autoclave was then opened to discharge thedispersion to the atmosphere to obtain expanded PP beads. The dischargewas carried out while feeding carbon dioxide gas such that the pressurewithin the autoclave was maintained at a pressure equal to the pressurein the autoclave immediately before the commencement of the discharge.The expanded PP beads were washed, centrifuged and allowed to stand inthe atmosphere for 48 hours for aging. The beads were then measured forheat of fusion of a high temperature peak thereof and melting point andhigh temperature peaks of surface and insides region thereof. Alsomeasured were MFR and apparent density of the beads. The results aresummarized in Tables 3-1 and 3-2. In Table 2, “1 Hr half lifetemperature” and “1 Min half life temperature” are as definedpreviously. TABLE 1 Glass Vicat Transition Softening MFR Melting ResinKind of Point Point (g/ Point No. Resin (° C.) (° C.) 10 min) (° C.) 1Propylene −21 148 8 163 homopolymer 2 Ethylene- −28 122 4 136 propylenerandom copolymer 3 Propylene −20 147 18 162 homopolymer

[0186] TABLE 2 Organic 1 Hr Half life 1 Min Half life PeroxideTemperature Temperature No. Organic Peroxide (° C.) (° C.) 1 Benzoylperoxide 92 130 2 Bis(4-t-butyl- 58  92 cyclohexyl) per- oxydicarbonate

[0187] The expanded PP beads were placed in a vessel, to whichpressurized air was fed so that the inside pressure of the cells of thebeads was increased to a pressure shown in Tables 3-1 and 3-2. Theresulting beads were then molded in the manner shown below using amolding device shown in FIG. 4 to obtain a foamed PP molding having theproperties shown in Tables 3-1 and 3-2.

[0188] The molding device had a male mold 1 and a female mold 2 adaptedto be displaced relative to each other. When the molds 1 and 2 arelocated in a fully closed position as shown in FIG. 4, a mold cavity 3having a size 15 of 250 mm×200 mm×50 mm was defined therebetween. Thedistance between the opposing inside walls of the molds 1 and 2, whichis indicated as “D” in FIG. 4, provides a thickness of a moldingproduced in the mold cavity 3 and equals 50 mm in the state shown inFIG. 4. After closing the molds 1 and 2 and drain valves 6 and 7, steamwas fed for 5 seconds through feed valves 4 and 5 each having a Cv value(indicative of flow rate characteristics thereof) of 18 and an effectivecross-sectional area of 320 mm² to warm up the molds. The mold 1 wasthen displaced relative to the mold 2 through a length of 1 mm (D wasincreased to 51 mm). Expanded PP beads were fed to the mold cavity 3and, thereafter, the molds 1 and 2 were closed again.

[0189] While maintaining each of the drain valves 6 and 7 in an openstate, the feed valves 4 and 5 were opened to feed steam into the moldcavity 3 through plenum chambers 1 a and 2 a and perforations (notshown) formed in the walls between the plenum chambers 1 a and 2 a andthe mold cavity 3 and to purge the air present between the beads fromthe cavity 3. The drain valves 6 and 7 were then closed and steam at apressure of 0.8 MPa(G) was fed through the feed valve 4 to the moldcavity 3 until a pressure lower by 0.04 MPa(G) than a predeterminedmolding pressure was reached in a pressure detecting line 9 connected toa pressure detecting device 11 (1st heating step). Next, while keepingthe drain valves 6 and 7 closed, steam at a pressure of 0.8 MPa(G) wasfed through the feed valve 5 to the mold cavity 3 until a pressure lowerby 0.02 MPa(G) than the predetermined molding pressure was reached in apressure detecting line 10 connected to a pressure detecting device 11(2nd heating step). A total process time in the 1st and 2nd heatingsteps was 20 seconds. Finally, while keeping the drain valves 6 and 7closed, steam was fed through the feed valves 4 and 5 to the mold cavity3 until the predetermined molding pressure was reached in each of thepressure detecting lines 9 and 10 (3rd, substantive heating step). Assoon as the predetermined molding pressure was reached, the feed valves4 and 5 were closed and the drain valves 6 and 7 were opened. The moldswere then cooled with water until a surface pressure on the molding of0.059 MPa(G) was reached in a pressure detecting line 8 connected to apressure detecting device 11. The molding was taken out of the moldcavity 3, aged at 60° C. for 24 hours and allowed to stand in a chamberat 23° C. for 14 days. Thereafter, physical properties of the moldingwere measured to give the results shown in Table 3-2.

[0190] The above-mentioned predetermined pressure of the saturated steamwas the minimum steam pressure P_(min) (MPa(G)) required for properlyfuse-bonding the beads to each other and determined by repeatedlyproducing moldings at various saturated steam pressures increasing from0.15 MPa(G) to 0.55 MPa(G) at an interval of 0.01 MPa(G). Thus, at apressure (P_(min)-0.01 MPa) , the beads were incapable of properlyfuse-bond together.

[0191] In determining the minimum steam pressure P_(min) required forproperly fuse-bonding the beads to each other, whether or not the beadswere properly bonded to each other was evaluated as follows:

[0192] A cut with a depth of 10 mm is formed on one of the two largestsides (250 mm×200 mm) of a sample of PP molding (size: 250 mm×200 mm×50mm) along a bisecting line perpendicular to the longitudinal directionthereof. The sample is then broken into halves along the cut line bybending. The interface along which the halves have been separated isobserved to count a total number C1 of the beads present on theinterface and the number C2 of the beads having destroyed cells. Whenthe ratio C2/C1 is at least 0.5, the sample is regarded as havingproperly fuse-bonded beads. The ratio C2/C1 increases with an increaseof the steam pressure. The minimum steam pressure P_(min) is a pressureat which the ratio C2/C1 is at least 0.5. At a pressure of (P_(min)-0.01MPa) , however, the ratio C2/C1 is lower than 0.5 and the beads areincapable of properly fuse-bond together.

[0193] The minimum steam pressure P_(min) is shown in Table 3-1 and 3-2.

[0194] The DSC analysis for the measurement of the physical propertiesof the polypropylene resin and the expanded PP beads was carried outusing Shimadzu Heat Flux Differential Scanning Calorimeter DSC-50(manufactured by SHIMADZU corporation) . Physical properties of the PPmoldings are also shown in Tables 3-1 and 3-2.

EXAMPLE 8

[0195] 100 Parts of polypropylene resin selected from those shown inTable 1 and indicated in Table 3-2 were blended with 0.05 part of zincborate powder (cell controlling agent) and the blend was kneaded in anextruder and extruded into strands. The strands were immediatelyintroduced in water at 18° C. for quenching. The cooled strands werethen cut into particles each having a length/diameter ratio of about 1.0and a mean weight of 2 mg.

[0196] In a 400 liter autoclave, 100 parts of the above resin particlesare dispersed in 120 parts of ion-exchanged water at 18° C. (weightratio of the resin particles to water of 0.83:1) together with 0.005part of sodium dodecylbenzenesulfonate (surfactant), 0.3 part of kaolinpowder (dispersing agent), 0.01 part of aluminum sulfate powder(dispersion enhancing agent), an organic peroxide selected from thoseshown in Table 2 and indicated in Table 3-2 in an amount shown in Table3-2. With stirring, the dispersion was heated at an average heating rateof 3° C. per minute to the 1 min half life temperature Tn (92° C.) ofthe peroxide and maintained at that temperature for 5 minutes tocomplete the decomposition of the peroxide. In this heating stage, thetime for which the dispersion was maintained at a temperature in therange of (Tn−30° C.) to (Tn+30° C.) was 15 minutes. Immediately afterthe above heat treatment, 100 parts of ion exchange water at 18° C. wereadded to the dispersion so that the weight ratio of the resin particlesto water was reduced to 0.45:1. Then, carbon dioxide (blowing agent) inan amount shown in 3-2 were added to the dispersion. The dispersion inthe autoclave was stirred, heated to a temperature lower by 5° C. thanthe expansion temperature shown in Table 3-2 at an average heating rateof 4° C./min and then maintained at that temperature for 15 min.Thereafter, the temperature was raised to the expansion temperature atan average heating rate of 3° C./min and maintained at that temperaturefor 15 min. One end of the autoclave was then opened to discharge thedispersion to the atmosphere to obtain expanded PP beads. The dischargewas carried out while feeding carbon dioxide gas such that the pressurewithin the autoclave was maintained at a pressure equal to the pressurein the autoclave immediately before the commencement of the discharge.The expanded PP beads were washed, centrifuged and allowed to stand inthe atmosphere at 23° C. for 48 hours for aging. The beads were thenmeasured for heat of fusion of a high temperature peak thereof andmelting point and high temperature peaks of surface and insides regionthereof. Also measured were MFR and apparent density of the beads. Theresults are summarized in Table 3-2. The expanded PP beads were found tobe substantially non-crosslinked (the boiling xylene insoluble contentwas 0).

[0197] The expanded PP beads thus obtained were placed in a vessel, towhich pressurized air was fed so that the inside pressure of the cellsof the beads was increased to a pressure shown in Table 3-2. The beadswere then molded in the same manner as that in Example 1 to obtain afoamed PP molding having the properties shown in Table 3-2.

EXAMPLE 9

[0198] 100 Parts of polypropylene resin selected from those shown inTable 1 and indicated in Table 3-2 were blended with 0.05 part of zincborate powder (cell controlling agent) and the blend was kneaded in anextruder and extruded into strands. The strands were immediatelyintroduced in water at 18° C. for quenching. The cooled strands werethen cut into particles each having a length/diameter ratio of about 1.0and a mean weight of 2 mg.

[0199] In a 400 liter autoclave, 100 parts of the above resin particlesare dispersed in 120 parts of ion-exchanged water at 18° C. (weightratio of the resin particles to water of 0.83:1) together with 0.005part of sodium dodecylbenzenesulfonate (surfactant), 0.4 part of kaolinpowder (dispersing agent), 0.013 part of aluminum sulfate powder(dispersion enhancing agent), an organic peroxide selected from thoseshown in Table 2 and indicated in Table 3-2 in an amount shown in Table3-2. With stirring, the dispersion was heated at an average heating rateof 50° C. per minute to 90° C. and maintained at that temperature for 10minutes to complete the decomposition of the peroxide. In this heatingstage, the time for which the dispersion was maintained at a temperaturein the range of (Tn−30° C.) to (Tn+30° C.) was 15 minutes. Immediatelyafter the above heat treatment, 100 parts of ion exchange water at 18°C. were added to the dispersion so that the weight ratio of the resinparticles to water was reduced to 0.45:1. Then, a high pressure carbondioxide gas (blowing agent) was charged in the autoclave until theinside pressure thereof was stabilized at 0.49 MPa(G). The dispersion inthe autoclave was then stirred, heated to a temperature lower by 5° C.than the expansion temperature shown in Table 3-2 at an average heatingrate of 5° C./min and maintained at that temperature for 15 min.Thereafter, the temperature was raised to a temperature lower by 1° C.than the expansion temperature at an average heating rate of 0.16°C./min. Subsequently, while a high pressure carbon dioxide gas (blowingagent) was charged in the autoclave until the inside pressure thereofwas stabilized at 1.18MPa(G), the temperature was raised to theexpansion temperature at an average heating rate of 0.029° C./min. Then,one end of the autoclave was then opened to discharge the dispersion tothe atmosphere to obtain expanded PP beads. The discharge was carriedout while feeding carbon dioxide gas such that the pressure within theautoclave was maintained at a pressure equal to the pressure in theautoclave immediately before the commencement of the discharge. Theexpanded PP beads were washed, centrifuged and allowed to stand in theatmosphere at 23° C. for 48 hours for aging. The beads were thenmeasured for heat of fusion of a high temperature peak thereof andmelting point and high temperature peaks of surface and insides regionthereof. Also measured were MFR and apparent density of the beads. Theresults are summarized in Table 3-2. The expanded PP beads were found tobe substantially non-crosslinked (the boiling xylene insoluble contentwas 0).

[0200] The expanded PP beads thus obtained were placed in an ambientpressure for 48 hours so that the inside pressure of the cells thereofwas equal to the ambient pressure. The resulting beads were then moldedin the manner shown below using a molding device shown in FIG. 4 toobtain a foamed PP molding having the properties shown in Table 3-2.

[0201] The molding device had a male mold 1 and a female mold 2 adaptedto be displaced relative to each other. When the molds 1 and 2 arelocated in a fully closed position as shown in FIG. 4, a mold cavity 3having a size of 250 mm×200 mm×50 mm was defined therebetween. Thedistance between the opposing inside walls of the molds 1 and 2, whichis indicated as “D” in FIG. 4, provides a thickness of a moldingproduced in the mold cavity 3 and equals 50 mm in the state shown inFIG. 4. After closing the molds 1 and 2 and drain valves 6 and 7, steamwas fed for 5 seconds through feed valves 4 and 5 each having a Cv value(indicative of flow rate characteristics thereof) of 18 and an effectivecross-sectional area of 320 mm² to warm up the molds. The mold 1 wasthen displaced relative to the mold 2 through a length of 10 mm (D wasincreased to 60 mm). Expanded PP beads were fed to the mold cavity 3and, thereafter, the molds 1 and 2 were closed again. While maintainingeach of the drain valves 6 and 7 in an open state, the feed valves 4 and5 were opened to feed steam into the mold cavity 3 through plenumchambers 1 a and 2 a and perforations (not shown) formed in the wallsbetween the plenum chambers 1 a and 2 a and the mold cavity 3 and topurge the air present between the beads from the cavity 3. The drainvalves 6 and 7 were then closed and steam at a pressure of 0.8 MPa(G)was fed through the feed valve 4 to the mold cavity 3 until a pressurelower by 0.04 MPa(G) than a predetermined molding pressure was reachedin a pressure detecting line 9 connected to a pressure detecting device11 (1st heating step). Next, while keeping the drain valves 6 and 7closed, steam at a pressure of 0.8 MPa(G) was fed through the feed valve5 to the mold cavity 3 until a pressure lower by 0.02 MPa(G) than thepredetermined molding pressure was reached in a pressure detecting line10 connected to a pressure detecting device 11 (2nd heating step).Finally, while keeping the drain valves 6 and 7 closed, steam was fedthrough the feed valves 4 and 5 to the mold cavity 3 until thepredetermined molding pressure was reached in each of the pressuredetecting lines 9 and 10 (3rd, substantive heating step). A totalprocess time in the 1st and 2nd heating steps was 12 seconds, while the3rd step was performed for 19 seconds. After the feed valves 4 and 5were closed and the drain valves 6 and 7 were opened, the molds werecooled with water until a surface pressure on the molding of 0.059MPa(G) was reached in a pressure detecting line 8 connected to apressure detecting device 11. The molding was taken out of the moldcavity 3, aged at 60° C. for 24 hours and allowed to stand in a chamberat 23° C. for 14 days. Thereafter, physical properties of the moldingwere measured to give the results shown in Table 3-2.

[0202] In the same manner as described above, foamed PP moldings wereproduced at various saturated steam pressures increasing from 0.15MPa(G) to 0.55 MPa(G) at an interval of 0.01 MPa(G) to determine theminimum steam pressure P_(min), The P_(min) thus determined is shown inTable 3-2. The data of the apparent density and the compression strengthof the PP molding of Example 9 shown in Table 3-2 are those of thefoamed PP molding obtained at a steam pressure of 0.41 MPa(G) ratherthan those at the minimum steam pressure P_(min). Although the PPmolding obtained at the minimum steam pressure P_(min) (0.039 MPa(G))shows good adhesion strength, the appearance of the PP molding was notsatisfactory in that a number of depressions were formed due toinsufficient inflation of the expanded PP beads. With a steam pressureof 0.41 MPa, on the other hand, no such depressions were found and thePP molding had good appearance. TABLE 3-1 Example 1 2 3 4 5 6Comparative Example Resin No. 1 1 1 1 1 2 particles MFR (g/10 min) 10 1010 10 10 7 Peroxide No. 1 2 2 2 2 2 Amount (part) 1 1 1 1 1 1 Expansiontemperature (° C.) 167.0 167.0 170.0 167.0 166.0 144.5 Amount of carbondioxide (part) 3 3 2.5 3 5.5 6.5 Apparent density of expanded 87 131 8987 78 48 PP beads (g/L) Melt of fusion of high whole 29.0 51.4 27.1 44.547.6 12.1 temperature peak (J/g) surface region 25.2 39.4 21.6 33.7 34.29.9 inside region 32.7 55.7 29.8 50.2 58.6 13.4 Melting point of surfaceregion 161.3 160.8 160.6 160.8 160.8 134.5 expanded PP beads insideregion 161.6 161.4 161.3 161.4 161.5 136.2 (° C.) MFR of expanded PPbeads (g/10 min) 30 23 22 24 23 18 Inside pressure of cells (MPa(G))0.23 0.29 0.16 0.29 0.35 0.12 Minimum steam pressure (MPa(G)) 0.48 0.440.35 0.38 0.39 0.17 Apparent density of PP molding (g/L) 55 91 58 53 4631 Apparent density of sample (g/L) 55 93 58 53 46 31 Compressionstrength (kPa) 570 1480 620 650 540 195

[0203] TABLE 3-2 Example Comparative Example 7 8 9 1 2 3 4 5 Resin No. 31 3 1 1 1 2 3 particles MFR (g/10 min) 18 10 19 10 11 10 7 18 PeroxideNo. 2 2 2 — — — — — Amount (part) 1 0.32 0.32 0 0 0 0 0 Expansiontemperature (° C.) 165.5 166.0 167.0 167.5 168.0 167.5 145.0 165.5Amount of carbon dioxide (part) 3 3 1 4.5 5 5 7 4 (*1) Apparent densityof expanded 85 128 125 131 69 83 48 93 PP beads (g/L) Heat of Fusionwhole 39.2 50.8 31.9 56.1 44.9 50.5 12.4 40.8 of high surface region20.8 45.9 20.0 51.5 41.6 46.4 11.5 39.4 temperature peak inside region45.0 59.6 41.4 58.7 47.8 52.7 12.8 40.8 (J/g) Melting point of surfaceregion 160.0 160.3 160.3 162.0 161.8 161.9 136.6 161.3 expanded PPinside region 160.6 161.1 160.7 161.5 161.6 161.6 136.2 160.6 beads (°C.) MFR of expanded PP beads 34 34 35 10 11 10 7 18 (g/10 min) Insidepressure of cells 0.19 0.31 0 0.29 0.35 0.29 0.12 0.50 (MPa(G)) Minimumsteam pressure 0.36 0.44 0.39 0.55 0.55 0.55 0.22 0.55 (MPa(G)) Apparentdensity of PP molding 54 90 91 91 46 54 31 61 (g/L) Apparent density ofsample (g/L) 53 92 93 91 46 54 31 61 Compression strength (kPa) 640 14601410 1359 510 650 195 790

[0204] In Tables 3-1 and 3-2, the apparent density (g/L) of the expandedPP beads is measured as follows. From the expanded PP beads which havebeen subjected to the aging, a quantity (0.5 to 10 g and 50 to 90 cm³)of the beads are arbitrarily selected. After the weight Wa (g) of theselected beads is measured, the beads are immersed in 100 ml of watercontained in a graduated measuring cylinder. From the volume increment,apparent volume Va (L) of the beads is determined. The apparent density(g/L) is calculated by dividing the weight Wa (g) of the beads by theapparent volume Va (L) of the beads. In Table 3-1 and 3-2, the symbol“(G)” is affixed to MPa to show that the pressure concerned is a gaugepressure.

[0205] In Comparative Examples 1-3 and 5, even when the maximumallowable pressure (0.55 MPa(g)) of the molding device was used, theC2/C1 ratios were 0, 0.16, 0.12 and 0.30, respectively, and lower than0.5. A higher pressure steam was thus needed to obtain PP moldingshaving properly fuse-bonded beads.

[0206] In Tables 3-1 and 3-2, the compression strength was measured asfollows. A PP molding was cut without leaving any outer surfaces thereofto obtain a sample having a size of 50 mm×50 mm×25 mm. The sample wassubjected to compression test in accordance with Japanese IndustrialStandard JIS Z0234-1976, A method. Thus, the sample was compressed at23° C. at a loading rate of 10 mm/min until a strain of 55% was reachedto obtain a stress-strain curve. The stress at 50% strain represents thecompression strength.

[0207] From the results shown in Tables 3-1 and 3-2, it is seen that theexpanded PP beads obtained from surface-modified propylene resinparticles give PP moldings having good recyclability and high mechanicalstrength at a relatively low molding temperature.

[0208] In particular, comparison of Example 2 with Comparative Example 1shows that they are almost the same with respect to the apparent densityof expanded PP beads, the heat of fusion of whole expanded PP bead, theapparent density of PP molding, and the apparent density of a PP moldingcut sample. However, the minimum pressure required for properlyfuse-bonding the beads to each other is more than 0.55 MPa(G) inComparative Example 1 and 0.44 MPa(G) in the case of Example 2,indicating that the minimum temperature required for fuse-bonding theexpanded PP beads of Example 2 is lower by at least 70° C. than that ofComparative Example 1. Yet, the mechanical strengths of the PP moldingof Example 2 are comparable to those of Comparative Example 1, asexpected from the similar heat of fusion of the high temperature peaksof the expanded PP beads of Comparative Example 1 and Example 2.

[0209] Comparison of Example 4 with Comparative Example 3 shows thatthey are almost the same with respect to the apparent density ofexpanded PP beads, the heat of fusion of whole expanded PP bead, theapparent density of PP molding, and the apparent density of a PP moldingcut sample. However, the minimum pressure required for properlyfuse-bonding the beads to each other is more than 0.55 MPa(G) inComparative Example 3 and 0.38 MPa(G) in the case of Example 4,indicating that the minimum temperature required for fuse-bonding of theexpanded PP beads of Example 4 is lower by at least 12° C. than that ofComparative Example 3. Yet, the mechanical strengths of the PP moldingof Example 4 are comparable to those of Comparative Example 3, asexpected from the similar heat of fusion of the high temperature peaksof the expanded PP beads of Comparative Example 3 and Example 4.

[0210] Comparison of Example 5 with Comparative Example 2 shows thatthey are almost the same with respect to the apparent density ofexpanded PP beads, the heat of fusion of whole expanded PP bead, theapparent density of PP molding, and the apparent density of a PP moldingcut sample. However, the minimum pressure required for properlyfuse-bonding the beads to each other is more than 0.55 MPa(G) inComparative Example 2 and 0.39 MPa(G) in the case of Example 5,indicating that the minimum temperature required for fuse-bonding of theexpanded PP beads of Example 5 is lower by at least 11° C. than that ofComparative Example 2. Yet, the mechanical strengths of the PP moldingof Example 5 are comparable to those of Comparative Example 2, asexpected from the similar heat of fusion of the high temperature peaksof the expanded PP beads of Comparative Example 2 and Example 5.

[0211] Comparison of Example 6 with Comparative Example 4 shows thatthey are almost the same with respect to the apparent density ofexpanded PP beads, the heat of fusion of whole expanded PP bead, theapparent density of PP molding, and the apparent density of a PP moldingcut sample. However, the minimum pressure required for properlyfuse-bonding the beads to each other is 0.22 MPa(G) in ComparativeExample 4 and 0.17 MPa(G) in the case of Example 6, indicating that theminimum temperature required for fuse-bonding of the expanded PP beadsof Example 6 is lower by at least 6° C. than that of Comparative Example4. Yet, the mechanical strengths of the PP molding of Example 6 arecomparable to those of Comparative Example 4, as expected from thesimilar heat of fusion of the high temperature peaks of the expanded PPbeads of Comparative Example 4 and Example 6.

[0212] Comparison of Example 1 with Example 3 shows that they are almostthe same with respect to the apparent density of expanded PP beads, theheat of fusion of whole expanded PP bead, the apparent density of PPmolding, and the apparent density of a PP molding cut sample. However,the minimum pressure required for properly fuse-bonding the beads toeach other is 0.48 MPa(G) in Example 1 and 0.35 MPa(G) in the case ofExample 3, indicating that the minimum temperature required forfuse-bonding of the expanded PP beads of Example 3 is lower by 9° C.than that of Example 1. Significant difference in the method ofproduction of expanded PP beads between Examples 1 and 3 is that Example3 uses a carbonate as an organic peroxide. Thus, the use of a carbonateis desirable for reasons of reduction of minimum temperature forfuse-bonding the expanded PP beads.

[0213] Comparison of Example 7 with Comparative Example 5 shows thatthey are almost the same with respect to the apparent density ofexpanded PP beads and the heat of fusion of whole expanded PP bead.Though these examples differ in the apparent density of PP molding andthe apparent density of a PP molding cut sample, such a difference wouldnot hinder fair comparison with respect to minimum pressure required forproperly fuse-bonding the beads to each other. Thus, the minimumpressure is more than 0.55 MPa(G) in Comparative Example 5 and 0.36MPa(G) in the case of Example 7, indicating that the minimum temperaturerequired for fuse-bonding of the expanded PP beads of Example 7 is lowerby at least 13° C. than that of Comparative Example 5. Higher mechanicalstrengths of the PP molding of Comparative Example 5 are as expectedfrom the higher heat of fusion of the high temperature peak of theexpanded PP beads of Comparative Example 5 and greater apparent densityof the PP molding of Comparative Example 5 as compared with those ofExample 7.

[0214] The micro differential thermoanalysis (μDTA) of the expanded PPbeads obtained in Example 7 and Comparative Example 5 was performedusing a micro differential thermoanalysis system (“Type 2990 MicroThermal Analyzer” of T. A. Instrument, Japan Inc.) at a heating rate of10° C./sec from 25° C. to 200° C. Such μDTA curves are shown in FIG. 3.It was found that the melt initiation temperature Pm and theextrapolated melt initiation temperature Pme were about 131° C. andabout 135° C., respectively, in the case of the expanded PP beads ofExample 7, whereas the melt initiation temperature Pnm and theextrapolated melt initiation temperature Pnme were about 168° C. andabout 171° C., respectively, in Comparative Example 5.

[0215] Thus, the low melt initiation temperature or the low extrapolatedmelt initiation temperature is considered to contribute the reduction ofthe minimum temperature required for fuse-bonding the expanded PP beadsof Example 7.

[0216] As compared with Example 2 in which the surface modification iscarried out with the weight ratio of the resin particles to thedispersing medium (water) of 0.45, Example 8 in which the particle/waterratio is 0.83 gives similar results in spite of the fact that theorganic peroxide is used in less amount (0.32 part) in Example 8 thanthat in Example 2 (1 part). In this connection, the advantage attainedin Example 8 is apparent in view of the fact that the apparent density,heat of fusion (entire) and inside pressure of cells of the expanded PPbeads of Examples 2 and 8 are similar and that the apparent density ofthe PP molding and the cut sample thereof in Examples 2 and 8 aresimilar.

[0217] In Example 9, the PP molding is prepared from expanded PP beadswhose inside pressure is equal to the ambient pressure. Such expandedbeads would require the use of much higher temperature steam in order toobtain a PP molding having good fusion between beads and good appearanceas compared with expanded beads whose cells have an increased insidepressure. By increasing the amount of the expanded beads filled in themold cavity and by adopting a three-step molding process in which the1st and 2nd heating steps are conducted for a relatively short period oftime and the 3rd, substantive heating step is carried out for arelatively long period of time, the PP molding obtained shows both goodadhesion between beads and good appearance even at a low steam pressureof 0.41 MPa. The melt initiation temperature Pm and the extrapolatedmelt initiation temperature Pme were found to be about 140° C. and about142° C., respectively, As described previously, a PP molding is regardedas having properly fuse-bonded beads, when the ratio C2/C1 is at least0.5. Table 4 shows relationships between C2/C1 ratios of PP moldings andsaturated steam pressures used for molding. As will be appreciated fromthe results shown in Table 4, a slight increase in saturated steampressure results in an increase of the C2/C1 ratio, namely increase ofthe bonding force between beads. A greater C2/C1 ratio is desirablebecause the PP molding has a higher resistance to fracture upon beingbent. TABLE 4 Saturated Steam C2/C1 Example No. Pressure (MPa(G)) RatioExample 1 0.48 0.51 0.49 0.65 Example 2 0.44 0.50 0.45 0.63 Example 30.35 0.52 0.37 0.80 Example 4 0.38 0.50 0.39 0.60 Example 5 0.39 0.530.41 0.66 Example 6 0.17 0.60 0.18 0.75 Example 7 0.36 0.54 0.37 0.60Example 8 0.44 0.56 0.45 0.70 Example 9 0.39 0.60 0.41 0.83 Comparative0.22 0.55 Example 4 0.23 0.62

EXAMPLES 10-14

[0218] 100 Parts of polypropylene resin selected from those shown inTable 5 and indicated in Table 10 were blended with a stericallyhindered amine compound selected from those shown in Table 6 in anamount shown in Table 10, an antioxidant selected from those shown inTable 9 in an amount shown in Table 10, 0.05 part of zinc borate powder(cell controlling agent) and, optionally, a UV-absorbing agent shown inTable 7 in an amount shown in Table 10. The blend was kneaded in anextruder and extruded into strands. The strands were immediatelyintroduced in water at 18° C. for quenching. The cooled strands werethen cut into particles each having a length/diameter ratio of about 1.0and a mean weight of 2 mg.

[0219] In a 5 liter autoclave, 100 parts of the above resin particlesare charged together with 300 parts of ion-exchanged water at 18° C.,0.004 part of sodium dodecylbenzenesulfonate (surfactant), 0.3 part ofkaolin powder (dispersing agent), 0.01 part of aluminum sulfate powder(dispersion enhancing agent), an organic peroxide selected from thoseshown in Table 8 and indicated in Table 10 in an amount shown in Table10, and carbon dioxide (blowing agent) in an amount shown in Table 10.The mixture in the autoclave was dispersed with stirring and heated to atemperature lower by 5° C. than the expansion temperature shown in Table10 at an average heating rate of 2° C./min and then maintained at thattemperature for 15 min. Thereafter, the temperature was raised to theexpansion temperature at an average heating rate of 1.5° C./min andmaintained at that temperature for 15 min. One end of the autoclave wasthen opened to discharge the dispersion to the atmosphere to obtainexpanded PP beads. The discharge was carried out while feeding nitrogengas such that the pressure within the autoclave was maintained at apressure equal to the pressure in the autoclave immediately before thecommencement of the discharge. The expanded PP beads were washed,centrifuged and allowed to stand in the atmosphere for 48 hours foraging. The beads were then measured for heat of fusion of a hightemperature peak thereof and melting point and high temperature peaks ofsurface and insides region thereof. Also measured were MFR and apparentdensity of the beads. The results are summarized in Table 10. Theexpanded PP beads were found to be substantially non-crosslinked (theboiling xylene insoluble content was 0). TABLE 5 Glass Vicat TransitionSoftening MFR Melting Resin Kind of Point Point (g/ Point No. Resin (°C.) (° C.) 10 min) (° C.) 3 Propylene −20 147 18 162 homopolymer 1Propylene −21 148 8 163 homopolymer 4 Propylene −20 150 30 166homopolymer

[0220] TABLE 6 Hindered Amine Molecular No. Name weight 1 Dimethylsuccinate-1-(2- 3500 hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine polycondensate 2 Bis(2,2,6,6-tetramethyl-4- 481piperidyl)sebacate 3 Poly[{6-(1,1,3,3-tetramethylbutyl)- 2500imino-1,3,5-triazine-2,4- diyl}{2,2,6,6-tetramethyl-4-piperidyl)imino}hexamethylene{2,2,6,6- tetramethyl-4-piperidyl)imino}]

[0221] TABLE 7 UV-Absorbing Agent Molecular No. Name weight 12-(3-t-Butyl-5-methyl-2- 316 hydroxyphenyl)-5- chlorobenzotriazol

[0222] The molecular weights of the Hindered Amine No. 1 and No. 3 andthe UV-Absorbing Agent No. 1 shown in Tables 6 and 7 are each averagemolecular weight (on polystyrene base) measured by gel permeationchromatography using GPC-LC3A (manufactured by SHIMADZU corporation) incombination with differential refraction detector RID-4 (manufactured bySHIMADZU corporation) as follows. A sample compound (10 mg) is dissolvedin a 10 ml mixed solvent composed of tetrahydrofuran, acetonitrile and10 mmol/L acetic acid The solution is loaded on a column (Shim-packCLC-ODS) having a diameter of 6 mm and a length of 150 mm. Thechromatography is carried out at a column temperature of 45° C. and anelution rate of 1 ml/min. TABLE 8 Organic 1 Hr Half life 1 Min Half lifePeroxide Temperature Temperature No. Organic Peroxide (° C.) (° C.) 2Bis(4-t-butyl- 58 92 cyclohexyl) per- oxydicarbonate

[0223] TABLE 9 Anti-oxidant Molecular No. Name weight 1pentaerythrytyl-tetrakis[3-(3,5-di-t- 1178butyl-4-hydroxyphenyl)propionate] 2 bis(2,4-di-t- 604butylphenyl)pentaerythrytol- diphosphite 3tris(2,4-di-t-butylphenyl)phosphite 647

[0224] The expanded PP beads were placed in a vessel, to whichpressurized air was fed so that the inside pressure of the cells of thebeads was increased to a pressure shown in Table 10. The beads were thenmolded in the same manner as that in Example 1 to obtain a foamed PPmoldings having the physical properties shown in Table 10. TABLE 10Example 10 11 12 13 14 Resin No. 3 1 1 4 4 particles MFR (g/10 min) 1810 10 30 30 Anti- No. 1 1 1 1 1 oxidant Amount (part) 0.03 0.025 0.0250.1 0.1 No. 2 3 3 3 3 Amount (part) 0.02 0.05 0.05 0.05 0.05 HinderedNo. 1 1 1 1 1 Amine Amount (part) 0.2 0.2 0.07 0.1 0.13 No. — — 2 3 —Amount (part) 0 0 0.13 0.1 0 UV- No. — — — — 1 absorber Amount (part) 00 0 0 0.07 Peroxide No. 2 2 2 2 2 Amount (part) 1 1 1 1 1 Expansiontemp. (° C.) 165.0 166.0 166.0 168.0 168.0 Amount of CO₂ (part) 3 3.53.5 2.5 2.5 Apparent density of 80 95 85 78 76 expanded PP beads (g/L)Melt of whole 36.7 41.7 41.0 38.9 39.9 fusion of surface 23.4 35.3 24.835.0 24.6 high region temperature inside 43.5 45.8 45.1 44.7 46.5 peak(J/g) region MFR (g/10 min) of 36 23 21 47 50 expanded PP beads Insidepressure of cells 0.25 0.39 0.23 0.25 0.32 of expanded PP beads (MPa(G))Minimum steam pressure 0.39 0.43 0.45 0.41 0.39 (MPa(G)) Apparentdensity of PP 54 63 55 53 54 molding (g/L) Compression strength 640 795660 630 650 (kPa) Weatherability A A A A A Heat resistance A A A A A

[0225] In Table 10, weatherability and heat resistance of the foamed PPmoldings are tested as follows.

[0226] Weatherability:

[0227] Using High Energy Xenon Weather Meter (Model SC750-WNmanufactured by Suga Test Machine Co., Ltd., light source: xenon longlife arc lamp, intensity of illumination: 90 W/m²), a sample issubjected to a weatherability test for 200 hours in accordance with JISD0205-1987. Evaluation is rated as follows.

[0228] A: no resin powder is generated even when the sample is rubbedwith finger

[0229] B: resin powder is generated when the sample is rubbed withfinger

[0230] Heat resistance:

[0231] A test sample is heated in a gear oven at 150° C. for 22 hours.Evaluation is rated as follows.

[0232] A: no resin powder is generated even when the sample is rubbedwith finger

[0233] B: resin powder is generated when the sample is rubbed withfinger

[0234] In one aspect, the present invention provides a process for theproduction of a foam PP molding, in which novel expanded PP beads whosesurfaces have been modified with an organic peroxide are filled in amold cavity and heated to be fuse-bonded together. The molding can becarried out at a significantly reduced temperature as compared with theconventional expanded PP beads. Thus, the process of the presentinvention can save consumption of thermal energy, reduce the coolingtime and improve the productivity. The PP molding produced issubstantially non-crosslinked and has good recyclability.

[0235] Hitherto, expanded PP beads showing a high temperatureendothermic peak in DSC curve thereof having high heat of fusion havebeen used for obtaining high rigidity, high impact strength PP moldings.In this case, a high molding temperature must be unavoidably employedand, hence, a general type molding machine designed to be used at usualpressure cannot be used. In contrast, according to the presentinvention, even when the expanded PP beads used are made of a highmelting point polypropylene resin and have high heat of fusion of hightemperature endothermic peak, the foamed PP molding having high rigiditycan be obtained using a low pressure steam (low temperature steam). Thispermits the use of the conventional general-type molding machine.Namely, the process of the present invention can produce PP moldingshaving high mechanical strengths and/or light weight at lower costs.

[0236] The foamed PP moldings obtained by the process of the presentinvention may be suitably used for various applications, for example, ascore materials for automobile bumpers and cushioning materials for sidedoors of automobiles.

[0237] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresent embodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all the changes which come within the meaning and rangeof equivalency of the claims are therefore intended to be embracedtherein.

[0238] The teachings of Japanese Patent Application No. 2001-336831,filed Nov. 1; 2001, No. 2001-401250, filed Dec. 28, 2001; and No.2002-054693, filed Feb. 28, 2002, inclusive of the specifications,claims and drawings, are hereby incorporated by reference herein.

In the claims:
 1. A process of producing a foamed molding, comprisingthe steps of: (a) dispersing substantially non-crosslinked resinparticles of a base resin including a polypropylene resin in adispersing medium containing an organic peroxide to obtain a dispersion;(b) maintaining said dispersion at a temperature lower than the meltingpoint of said base resin but sufficient to decompose said organicperoxide, thereby obtaining substantially non-crosslinked,surface-modified resin particles; (c) foaming and expanding saidnon-crosslinked, surface-modified resin particles using a blowing agentto obtain expanded, substantially non-crosslinked resin beads; (d)filling said expanded resin beads in a mold cavity; (e) heating saidexpanded resin beads in said mold cavity to fuse-bond said expandedresin beads together into a unitary body; and (f) cooling said unitarybody.
 2. A process of producing a foamed molding, comprising the stepsof: (a) filling expanded, substantially non-crosslinked resin beads of abase resin including a polypropylene resin in a mold cavity, saidexpanded resin beads having a surface region and an inside regionsurrounded by said surface region, wherein each of said surface andinside regions shows a high temperature endothermic peak, in a DSC curvethereof, in addition to an intrinsic endothermic peak located at a lowertemperature side of said high temperature peak, and wherein said hightemperature endothermic peaks of said surface region and said insideregion have heat of fusion of Hs and Hi, respectively, and wherein Hsand Hi have the following relationship: Hs<0.86×Hi (b) heating saidexpanded resin beads in said mold cavity to fuse-bond said expandedresin beads together into a unitary body; and (c) cooling said unitarybody.
 3. A process of producing a foamed molding, comprising the stepsof: (a) filling expanded, substantially non-crosslinked resin beads of abase resin including a polypropylene resin in a mold cavity, saidexpanded resin beads exhibiting a high temperature endothermic peak, ina DSC curve thereof, in addition to an intrinsic endothermic peaklocated at a lower temperature side of said high temperature peak, eachof said expanded beads having a surface showing a melt initiationtemperature not higher than the melting point of said base resin; (b)heating said expanded resin beads in said mold cavity to fuse-bond saidexpanded resin beads together into a unitary body; and (c) cooling saidunitary body.
 4. A process of producing a foamed molding, comprising thesteps of: (a) filling expanded, substantially non-crosslinked resinbeads of a base resin including a polypropylene resin in a mold cavity,said expanded resin beads exhibiting a high temperature endothermicpeak, in a DSC curve thereof, in addition to an intrinsic endothermicpeak located at a lower temperature side of said high temperature peak,each of said expanded beads having a surface having an extrapolated meltinitiation temperature, as measured by micro differentialthermoanalysis, not higher than (Tm+4° C.) where Tm is the melting pointof the base resin, (b) heating said expanded resin beads in said moldcavity to fuse-bond said expanded resin beads together into a unitarybody; and (c) cooling said unitary body.
 5. A process as claimed inclaim 2, wherein said surface region has a melting point lower than thatof said inside region.
 6. A process as claimed in claim 1, wherein saidexpanded resin beads have a surface region and an inside regionsurrounded by said surface region, and wherein said surface region has amelting point lower than that of said inside region.
 7. A process asclaimed in claim 1, wherein said base resin has a melting point of atleast 158° C.
 8. A process as claimed in claim 1, wherein the heat offusion of said high temperature endothermic peak ranges from 2 to 70J/g.
 9. A process as claimed in claim 1, wherein said expanded resinbeads comprise a hindered amine compound.
 10. A process for thepreparation of expanded resin beads, comprising the steps of: (a)dispersing substantially non-crosslinked resin particles of a base resinincluding a polypropylene resin in a dispersing medium containing anorganic peroxide to obtain a first dispersion having a weight ratio ofsaid resin, particles to said dispersing medium of 0.6:1 to 1.3: 1; (b)maintaining said dispersion at a temperature lower than the meltingpoint of said base resin but sufficient to decompose said organicperoxide, thereby obtaining substantially non-crosslinked,surface-modified resin particles dispersed in said dispersing medium;(c) impregnating said surface-modified resin particles with a blowingagent, while maintaining the weight ratio of said surface-modified resinparticles to said dispersing medium at 0.5 or less, to obtain a seconddispersion; and (d) reducing the pressure of said second dispersion tofoam and expand said surface-modified resin particles.